Source file src/cmd/go/internal/work/buildid.go
1 // Copyright 2017 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package work 6 7 import ( 8 "bytes" 9 "fmt" 10 "os" 11 "os/exec" 12 "strings" 13 14 "cmd/go/internal/base" 15 "cmd/go/internal/cache" 16 "cmd/go/internal/cfg" 17 "cmd/go/internal/fsys" 18 "cmd/go/internal/str" 19 "cmd/internal/buildid" 20 "cmd/internal/quoted" 21 ) 22 23 // Build IDs 24 // 25 // Go packages and binaries are stamped with build IDs that record both 26 // the action ID, which is a hash of the inputs to the action that produced 27 // the packages or binary, and the content ID, which is a hash of the action 28 // output, namely the archive or binary itself. The hash is the same one 29 // used by the build artifact cache (see cmd/go/internal/cache), but 30 // truncated when stored in packages and binaries, as the full length is not 31 // needed and is a bit unwieldy. The precise form is 32 // 33 // actionID/[.../]contentID 34 // 35 // where the actionID and contentID are prepared by buildid.HashToString below. 36 // and are found by looking for the first or last slash. 37 // Usually the buildID is simply actionID/contentID, but see below for an 38 // exception. 39 // 40 // The build ID serves two primary purposes. 41 // 42 // 1. The action ID half allows installed packages and binaries to serve as 43 // one-element cache entries. If we intend to build math.a with a given 44 // set of inputs summarized in the action ID, and the installed math.a already 45 // has that action ID, we can reuse the installed math.a instead of rebuilding it. 46 // 47 // 2. The content ID half allows the easy preparation of action IDs for steps 48 // that consume a particular package or binary. The content hash of every 49 // input file for a given action must be included in the action ID hash. 50 // Storing the content ID in the build ID lets us read it from the file with 51 // minimal I/O, instead of reading and hashing the entire file. 52 // This is especially effective since packages and binaries are typically 53 // the largest inputs to an action. 54 // 55 // Separating action ID from content ID is important for reproducible builds. 56 // The compiler is compiled with itself. If an output were represented by its 57 // own action ID (instead of content ID) when computing the action ID of 58 // the next step in the build process, then the compiler could never have its 59 // own input action ID as its output action ID (short of a miraculous hash collision). 60 // Instead we use the content IDs to compute the next action ID, and because 61 // the content IDs converge, so too do the action IDs and therefore the 62 // build IDs and the overall compiler binary. See cmd/dist's cmdbootstrap 63 // for the actual convergence sequence. 64 // 65 // The “one-element cache” purpose is a bit more complex for installed 66 // binaries. For a binary, like cmd/gofmt, there are two steps: compile 67 // cmd/gofmt/*.go into main.a, and then link main.a into the gofmt binary. 68 // We do not install gofmt's main.a, only the gofmt binary. Being able to 69 // decide that the gofmt binary is up-to-date means computing the action ID 70 // for the final link of the gofmt binary and comparing it against the 71 // already-installed gofmt binary. But computing the action ID for the link 72 // means knowing the content ID of main.a, which we did not keep. 73 // To sidestep this problem, each binary actually stores an expanded build ID: 74 // 75 // actionID(binary)/actionID(main.a)/contentID(main.a)/contentID(binary) 76 // 77 // (Note that this can be viewed equivalently as: 78 // 79 // actionID(binary)/buildID(main.a)/contentID(binary) 80 // 81 // Storing the buildID(main.a) in the middle lets the computations that care 82 // about the prefix or suffix halves ignore the middle and preserves the 83 // original build ID as a contiguous string.) 84 // 85 // During the build, when it's time to build main.a, the gofmt binary has the 86 // information needed to decide whether the eventual link would produce 87 // the same binary: if the action ID for main.a's inputs matches and then 88 // the action ID for the link step matches when assuming the given main.a 89 // content ID, then the binary as a whole is up-to-date and need not be rebuilt. 90 // 91 // This is all a bit complex and may be simplified once we can rely on the 92 // main cache, but at least at the start we will be using the content-based 93 // staleness determination without a cache beyond the usual installed 94 // package and binary locations. 95 96 const buildIDSeparator = "/" 97 98 // actionID returns the action ID half of a build ID. 99 func actionID(buildID string) string { 100 i := strings.Index(buildID, buildIDSeparator) 101 if i < 0 { 102 return buildID 103 } 104 return buildID[:i] 105 } 106 107 // contentID returns the content ID half of a build ID. 108 func contentID(buildID string) string { 109 return buildID[strings.LastIndex(buildID, buildIDSeparator)+1:] 110 } 111 112 // toolID returns the unique ID to use for the current copy of the 113 // named tool (asm, compile, cover, link). 114 // 115 // It is important that if the tool changes (for example a compiler bug is fixed 116 // and the compiler reinstalled), toolID returns a different string, so that old 117 // package archives look stale and are rebuilt (with the fixed compiler). 118 // This suggests using a content hash of the tool binary, as stored in the build ID. 119 // 120 // Unfortunately, we can't just open the tool binary, because the tool might be 121 // invoked via a wrapper program specified by -toolexec and we don't know 122 // what the wrapper program does. In particular, we want "-toolexec toolstash" 123 // to continue working: it does no good if "-toolexec toolstash" is executing a 124 // stashed copy of the compiler but the go command is acting as if it will run 125 // the standard copy of the compiler. The solution is to ask the tool binary to tell 126 // us its own build ID using the "-V=full" flag now supported by all tools. 127 // Then we know we're getting the build ID of the compiler that will actually run 128 // during the build. (How does the compiler binary know its own content hash? 129 // We store it there using updateBuildID after the standard link step.) 130 // 131 // A final twist is that we'd prefer to have reproducible builds for release toolchains. 132 // It should be possible to cross-compile for Windows from either Linux or Mac 133 // or Windows itself and produce the same binaries, bit for bit. If the tool ID, 134 // which influences the action ID half of the build ID, is based on the content ID, 135 // then the Linux compiler binary and Mac compiler binary will have different tool IDs 136 // and therefore produce executables with different action IDs. 137 // To avoid this problem, for releases we use the release version string instead 138 // of the compiler binary's content hash. This assumes that all compilers built 139 // on all different systems are semantically equivalent, which is of course only true 140 // modulo bugs. (Producing the exact same executables also requires that the different 141 // build setups agree on details like $GOROOT and file name paths, but at least the 142 // tool IDs do not make it impossible.) 143 func (b *Builder) toolID(name string) string { 144 b.id.Lock() 145 id := b.toolIDCache[name] 146 b.id.Unlock() 147 148 if id != "" { 149 return id 150 } 151 152 path := base.Tool(name) 153 desc := "go tool " + name 154 155 // Special case: undocumented -vettool overrides usual vet, 156 // for testing vet or supplying an alternative analysis tool. 157 if name == "vet" && VetTool != "" { 158 path = VetTool 159 desc = VetTool 160 } 161 162 cmdline := str.StringList(cfg.BuildToolexec, path, "-V=full") 163 cmd := exec.Command(cmdline[0], cmdline[1:]...) 164 var stdout, stderr strings.Builder 165 cmd.Stdout = &stdout 166 cmd.Stderr = &stderr 167 if err := cmd.Run(); err != nil { 168 if stderr.Len() > 0 { 169 os.Stderr.WriteString(stderr.String()) 170 } 171 base.Fatalf("go: error obtaining buildID for %s: %v", desc, err) 172 } 173 174 line := stdout.String() 175 f := strings.Fields(line) 176 if len(f) < 3 || f[0] != name && path != VetTool || f[1] != "version" || f[2] == "devel" && !strings.HasPrefix(f[len(f)-1], "buildID=") { 177 base.Fatalf("go: parsing buildID from %s -V=full: unexpected output:\n\t%s", desc, line) 178 } 179 if f[2] == "devel" { 180 // On the development branch, use the content ID part of the build ID. 181 id = contentID(f[len(f)-1]) 182 } else { 183 // For a release, the output is like: "compile version go1.9.1 X:framepointer". 184 // Use the whole line. 185 id = strings.TrimSpace(line) 186 } 187 188 b.id.Lock() 189 b.toolIDCache[name] = id 190 b.id.Unlock() 191 192 return id 193 } 194 195 // gccToolID returns the unique ID to use for a tool that is invoked 196 // by the GCC driver. This is used particularly for gccgo, but this can also 197 // be used for gcc, g++, gfortran, etc.; those tools all use the GCC 198 // driver under different names. The approach used here should also 199 // work for sufficiently new versions of clang. Unlike toolID, the 200 // name argument is the program to run. The language argument is the 201 // type of input file as passed to the GCC driver's -x option. 202 // 203 // For these tools we have no -V=full option to dump the build ID, 204 // but we can run the tool with -v -### to reliably get the compiler proper 205 // and hash that. That will work in the presence of -toolexec. 206 // 207 // In order to get reproducible builds for released compilers, we 208 // detect a released compiler by the absence of "experimental" in the 209 // --version output, and in that case we just use the version string. 210 // 211 // gccToolID also returns the underlying executable for the compiler. 212 // The caller assumes that stat of the exe can be used, combined with the id, 213 // to detect changes in the underlying compiler. The returned exe can be empty, 214 // which means to rely only on the id. 215 func (b *Builder) gccToolID(name, language string) (id, exe string, err error) { 216 key := name + "." + language 217 b.id.Lock() 218 id = b.toolIDCache[key] 219 exe = b.toolIDCache[key+".exe"] 220 b.id.Unlock() 221 222 if id != "" { 223 return id, exe, nil 224 } 225 226 // Invoke the driver with -### to see the subcommands and the 227 // version strings. Use -x to set the language. Pretend to 228 // compile an empty file on standard input. 229 cmdline := str.StringList(cfg.BuildToolexec, name, "-###", "-x", language, "-c", "-") 230 cmd := exec.Command(cmdline[0], cmdline[1:]...) 231 // Force untranslated output so that we see the string "version". 232 cmd.Env = append(os.Environ(), "LC_ALL=C") 233 out, err := cmd.CombinedOutput() 234 if err != nil { 235 return "", "", fmt.Errorf("%s: %v; output: %q", name, err, out) 236 } 237 238 version := "" 239 lines := strings.Split(string(out), "\n") 240 for _, line := range lines { 241 fields := strings.Fields(line) 242 for i, field := range fields { 243 if strings.HasSuffix(field, ":") { 244 // Avoid parsing fields of lines like "Configured with: …", which may 245 // contain arbitrary substrings. 246 break 247 } 248 if field == "version" && i < len(fields)-1 { 249 // Check that the next field is plausibly a version number. 250 // We require only that it begins with an ASCII digit, 251 // since we don't know what version numbering schemes a given 252 // C compiler may use. (Clang and GCC mostly seem to follow the scheme X.Y.Z, 253 // but in https://go.dev/issue/64619 we saw "8.3 [DragonFly]", and who knows 254 // what other C compilers like "zig cc" might report?) 255 next := fields[i+1] 256 if len(next) > 0 && next[0] >= '0' && next[0] <= '9' { 257 version = line 258 break 259 } 260 } 261 } 262 if version != "" { 263 break 264 } 265 } 266 if version == "" { 267 return "", "", fmt.Errorf("%s: can not find version number in %q", name, out) 268 } 269 270 if !strings.Contains(version, "experimental") { 271 // This is a release. Use this line as the tool ID. 272 id = version 273 } else { 274 // This is a development version. The first line with 275 // a leading space is the compiler proper. 276 compiler := "" 277 for _, line := range lines { 278 if strings.HasPrefix(line, " ") && !strings.HasPrefix(line, " (in-process)") { 279 compiler = line 280 break 281 } 282 } 283 if compiler == "" { 284 return "", "", fmt.Errorf("%s: can not find compilation command in %q", name, out) 285 } 286 287 fields, _ := quoted.Split(compiler) 288 if len(fields) == 0 { 289 return "", "", fmt.Errorf("%s: compilation command confusion %q", name, out) 290 } 291 exe = fields[0] 292 if !strings.ContainsAny(exe, `/\`) { 293 if lp, err := cfg.LookPath(exe); err == nil { 294 exe = lp 295 } 296 } 297 id, err = buildid.ReadFile(exe) 298 if err != nil { 299 return "", "", err 300 } 301 302 // If we can't find a build ID, use a hash. 303 if id == "" { 304 id = b.fileHash(exe) 305 } 306 } 307 308 b.id.Lock() 309 b.toolIDCache[key] = id 310 b.toolIDCache[key+".exe"] = exe 311 b.id.Unlock() 312 313 return id, exe, nil 314 } 315 316 // Check if assembler used by gccgo is GNU as. 317 func assemblerIsGas() bool { 318 cmd := exec.Command(BuildToolchain.compiler(), "-print-prog-name=as") 319 assembler, err := cmd.Output() 320 if err == nil { 321 cmd := exec.Command(strings.TrimSpace(string(assembler)), "--version") 322 out, err := cmd.Output() 323 return err == nil && strings.Contains(string(out), "GNU") 324 } else { 325 return false 326 } 327 } 328 329 // gccgoBuildIDFile creates an assembler file that records the 330 // action's build ID in an SHF_EXCLUDE section for ELF files or 331 // in a CSECT in XCOFF files. 332 func (b *Builder) gccgoBuildIDFile(a *Action) (string, error) { 333 sfile := a.Objdir + "_buildid.s" 334 335 var buf bytes.Buffer 336 if cfg.Goos == "aix" { 337 fmt.Fprintf(&buf, "\t.csect .go.buildid[XO]\n") 338 } else if (cfg.Goos != "solaris" && cfg.Goos != "illumos") || assemblerIsGas() { 339 fmt.Fprintf(&buf, "\t"+`.section .go.buildid,"e"`+"\n") 340 } else if cfg.Goarch == "sparc" || cfg.Goarch == "sparc64" { 341 fmt.Fprintf(&buf, "\t"+`.section ".go.buildid",#exclude`+"\n") 342 } else { // cfg.Goarch == "386" || cfg.Goarch == "amd64" 343 fmt.Fprintf(&buf, "\t"+`.section .go.buildid,#exclude`+"\n") 344 } 345 fmt.Fprintf(&buf, "\t.byte ") 346 for i := 0; i < len(a.buildID); i++ { 347 if i > 0 { 348 if i%8 == 0 { 349 fmt.Fprintf(&buf, "\n\t.byte ") 350 } else { 351 fmt.Fprintf(&buf, ",") 352 } 353 } 354 fmt.Fprintf(&buf, "%#02x", a.buildID[i]) 355 } 356 fmt.Fprintf(&buf, "\n") 357 if cfg.Goos != "solaris" && cfg.Goos != "illumos" && cfg.Goos != "aix" { 358 secType := "@progbits" 359 if cfg.Goarch == "arm" { 360 secType = "%progbits" 361 } 362 fmt.Fprintf(&buf, "\t"+`.section .note.GNU-stack,"",%s`+"\n", secType) 363 fmt.Fprintf(&buf, "\t"+`.section .note.GNU-split-stack,"",%s`+"\n", secType) 364 } 365 366 if err := b.Shell(a).writeFile(sfile, buf.Bytes()); err != nil { 367 return "", err 368 } 369 370 return sfile, nil 371 } 372 373 // buildID returns the build ID found in the given file. 374 // If no build ID is found, buildID returns the content hash of the file. 375 func (b *Builder) buildID(file string) string { 376 b.id.Lock() 377 id := b.buildIDCache[file] 378 b.id.Unlock() 379 380 if id != "" { 381 return id 382 } 383 384 id, err := buildid.ReadFile(file) 385 if err != nil { 386 id = b.fileHash(file) 387 } 388 389 b.id.Lock() 390 b.buildIDCache[file] = id 391 b.id.Unlock() 392 393 return id 394 } 395 396 // fileHash returns the content hash of the named file. 397 func (b *Builder) fileHash(file string) string { 398 file, _ = fsys.OverlayPath(file) 399 sum, err := cache.FileHash(file) 400 if err != nil { 401 return "" 402 } 403 return buildid.HashToString(sum) 404 } 405 406 // useCache tries to satisfy the action a, which has action ID actionHash, 407 // by using a cached result from an earlier build. At the moment, the only 408 // cached result is the installed package or binary at target. 409 // If useCache decides that the cache can be used, it sets a.buildID 410 // and a.built for use by parent actions and then returns true. 411 // Otherwise it sets a.buildID to a temporary build ID for use in the build 412 // and returns false. When useCache returns false the expectation is that 413 // the caller will build the target and then call updateBuildID to finish the 414 // build ID computation. 415 // When useCache returns false, it may have initiated buffering of output 416 // during a's work. The caller should defer b.flushOutput(a), to make sure 417 // that flushOutput is eventually called regardless of whether the action 418 // succeeds. The flushOutput call must happen after updateBuildID. 419 func (b *Builder) useCache(a *Action, actionHash cache.ActionID, target string, printOutput bool) bool { 420 // The second half of the build ID here is a placeholder for the content hash. 421 // It's important that the overall buildID be unlikely verging on impossible 422 // to appear in the output by chance, but that should be taken care of by 423 // the actionID half; if it also appeared in the input that would be like an 424 // engineered 120-bit partial SHA256 collision. 425 a.actionID = actionHash 426 actionID := buildid.HashToString(actionHash) 427 if a.json != nil { 428 a.json.ActionID = actionID 429 } 430 contentID := actionID // temporary placeholder, likely unique 431 a.buildID = actionID + buildIDSeparator + contentID 432 433 // Executable binaries also record the main build ID in the middle. 434 // See "Build IDs" comment above. 435 if a.Mode == "link" { 436 mainpkg := a.Deps[0] 437 a.buildID = actionID + buildIDSeparator + mainpkg.buildID + buildIDSeparator + contentID 438 } 439 440 // If user requested -a, we force a rebuild, so don't use the cache. 441 if cfg.BuildA { 442 if p := a.Package; p != nil && !p.Stale { 443 p.Stale = true 444 p.StaleReason = "build -a flag in use" 445 } 446 // Begin saving output for later writing to cache. 447 a.output = []byte{} 448 return false 449 } 450 451 c := cache.Default() 452 453 if target != "" { 454 buildID, _ := buildid.ReadFile(target) 455 if strings.HasPrefix(buildID, actionID+buildIDSeparator) { 456 a.buildID = buildID 457 if a.json != nil { 458 a.json.BuildID = a.buildID 459 } 460 a.built = target 461 // Poison a.Target to catch uses later in the build. 462 a.Target = "DO NOT USE - " + a.Mode 463 return true 464 } 465 // Special case for building a main package: if the only thing we 466 // want the package for is to link a binary, and the binary is 467 // already up-to-date, then to avoid a rebuild, report the package 468 // as up-to-date as well. See "Build IDs" comment above. 469 // TODO(rsc): Rewrite this code to use a TryCache func on the link action. 470 if !b.NeedExport && a.Mode == "build" && len(a.triggers) == 1 && a.triggers[0].Mode == "link" { 471 if id := strings.Split(buildID, buildIDSeparator); len(id) == 4 && id[1] == actionID { 472 // Temporarily assume a.buildID is the package build ID 473 // stored in the installed binary, and see if that makes 474 // the upcoming link action ID a match. If so, report that 475 // we built the package, safe in the knowledge that the 476 // link step will not ask us for the actual package file. 477 // Note that (*Builder).LinkAction arranged that all of 478 // a.triggers[0]'s dependencies other than a are also 479 // dependencies of a, so that we can be sure that, 480 // other than a.buildID, b.linkActionID is only accessing 481 // build IDs of completed actions. 482 oldBuildID := a.buildID 483 a.buildID = id[1] + buildIDSeparator + id[2] 484 linkID := buildid.HashToString(b.linkActionID(a.triggers[0])) 485 if id[0] == linkID { 486 // Best effort attempt to display output from the compile and link steps. 487 // If it doesn't work, it doesn't work: reusing the cached binary is more 488 // important than reprinting diagnostic information. 489 if printOutput { 490 showStdout(b, c, a, "stdout") // compile output 491 showStdout(b, c, a, "link-stdout") // link output 492 } 493 494 // Poison a.Target to catch uses later in the build. 495 a.Target = "DO NOT USE - main build pseudo-cache Target" 496 a.built = "DO NOT USE - main build pseudo-cache built" 497 if a.json != nil { 498 a.json.BuildID = a.buildID 499 } 500 return true 501 } 502 // Otherwise restore old build ID for main build. 503 a.buildID = oldBuildID 504 } 505 } 506 } 507 508 // Special case for linking a test binary: if the only thing we 509 // want the binary for is to run the test, and the test result is cached, 510 // then to avoid the link step, report the link as up-to-date. 511 // We avoid the nested build ID problem in the previous special case 512 // by recording the test results in the cache under the action ID half. 513 if len(a.triggers) == 1 && a.triggers[0].TryCache != nil && a.triggers[0].TryCache(b, a.triggers[0]) { 514 // Best effort attempt to display output from the compile and link steps. 515 // If it doesn't work, it doesn't work: reusing the test result is more 516 // important than reprinting diagnostic information. 517 if printOutput { 518 showStdout(b, c, a.Deps[0], "stdout") // compile output 519 showStdout(b, c, a.Deps[0], "link-stdout") // link output 520 } 521 522 // Poison a.Target to catch uses later in the build. 523 a.Target = "DO NOT USE - pseudo-cache Target" 524 a.built = "DO NOT USE - pseudo-cache built" 525 return true 526 } 527 528 // Check to see if the action output is cached. 529 if file, _, err := cache.GetFile(c, actionHash); err == nil { 530 if buildID, err := buildid.ReadFile(file); err == nil { 531 if printOutput { 532 showStdout(b, c, a, "stdout") 533 } 534 a.built = file 535 a.Target = "DO NOT USE - using cache" 536 a.buildID = buildID 537 if a.json != nil { 538 a.json.BuildID = a.buildID 539 } 540 if p := a.Package; p != nil && target != "" { 541 p.Stale = true 542 // Clearer than explaining that something else is stale. 543 p.StaleReason = "not installed but available in build cache" 544 } 545 return true 546 } 547 } 548 549 // If we've reached this point, we can't use the cache for the action. 550 if p := a.Package; p != nil && !p.Stale { 551 p.Stale = true 552 p.StaleReason = "build ID mismatch" 553 if b.IsCmdList { 554 // Since we may end up printing StaleReason, include more detail. 555 for _, p1 := range p.Internal.Imports { 556 if p1.Stale && p1.StaleReason != "" { 557 if strings.HasPrefix(p1.StaleReason, "stale dependency: ") { 558 p.StaleReason = p1.StaleReason 559 break 560 } 561 if strings.HasPrefix(p.StaleReason, "build ID mismatch") { 562 p.StaleReason = "stale dependency: " + p1.ImportPath 563 } 564 } 565 } 566 } 567 } 568 569 // Begin saving output for later writing to cache. 570 a.output = []byte{} 571 return false 572 } 573 574 func showStdout(b *Builder, c cache.Cache, a *Action, key string) error { 575 actionID := a.actionID 576 577 stdout, stdoutEntry, err := cache.GetBytes(c, cache.Subkey(actionID, key)) 578 if err != nil { 579 return err 580 } 581 582 if len(stdout) > 0 { 583 sh := b.Shell(a) 584 if cfg.BuildX || cfg.BuildN { 585 sh.ShowCmd("", "%s # internal", joinUnambiguously(str.StringList("cat", c.OutputFile(stdoutEntry.OutputID)))) 586 } 587 if !cfg.BuildN { 588 sh.Print(string(stdout)) 589 } 590 } 591 return nil 592 } 593 594 // flushOutput flushes the output being queued in a. 595 func (b *Builder) flushOutput(a *Action) { 596 b.Shell(a).Print(string(a.output)) 597 a.output = nil 598 } 599 600 // updateBuildID updates the build ID in the target written by action a. 601 // It requires that useCache was called for action a and returned false, 602 // and that the build was then carried out and given the temporary 603 // a.buildID to record as the build ID in the resulting package or binary. 604 // updateBuildID computes the final content ID and updates the build IDs 605 // in the binary. 606 // 607 // Keep in sync with src/cmd/buildid/buildid.go 608 func (b *Builder) updateBuildID(a *Action, target string, rewrite bool) error { 609 sh := b.Shell(a) 610 611 if cfg.BuildX || cfg.BuildN { 612 if rewrite { 613 sh.ShowCmd("", "%s # internal", joinUnambiguously(str.StringList(base.Tool("buildid"), "-w", target))) 614 } 615 if cfg.BuildN { 616 return nil 617 } 618 } 619 620 c := cache.Default() 621 622 // Cache output from compile/link, even if we don't do the rest. 623 switch a.Mode { 624 case "build": 625 cache.PutBytes(c, cache.Subkey(a.actionID, "stdout"), a.output) 626 case "link": 627 // Even though we don't cache the binary, cache the linker text output. 628 // We might notice that an installed binary is up-to-date but still 629 // want to pretend to have run the linker. 630 // Store it under the main package's action ID 631 // to make it easier to find when that's all we have. 632 for _, a1 := range a.Deps { 633 if p1 := a1.Package; p1 != nil && p1.Name == "main" { 634 cache.PutBytes(c, cache.Subkey(a1.actionID, "link-stdout"), a.output) 635 break 636 } 637 } 638 } 639 640 // Find occurrences of old ID and compute new content-based ID. 641 r, err := os.Open(target) 642 if err != nil { 643 return err 644 } 645 matches, hash, err := buildid.FindAndHash(r, a.buildID, 0) 646 r.Close() 647 if err != nil { 648 return err 649 } 650 newID := a.buildID[:strings.LastIndex(a.buildID, buildIDSeparator)] + buildIDSeparator + buildid.HashToString(hash) 651 if len(newID) != len(a.buildID) { 652 return fmt.Errorf("internal error: build ID length mismatch %q vs %q", a.buildID, newID) 653 } 654 655 // Replace with new content-based ID. 656 a.buildID = newID 657 if a.json != nil { 658 a.json.BuildID = a.buildID 659 } 660 if len(matches) == 0 { 661 // Assume the user specified -buildid= to override what we were going to choose. 662 return nil 663 } 664 665 if rewrite { 666 w, err := os.OpenFile(target, os.O_RDWR, 0) 667 if err != nil { 668 return err 669 } 670 err = buildid.Rewrite(w, matches, newID) 671 if err != nil { 672 w.Close() 673 return err 674 } 675 if err := w.Close(); err != nil { 676 return err 677 } 678 } 679 680 // Cache package builds, but not binaries (link steps). 681 // The expectation is that binaries are not reused 682 // nearly as often as individual packages, and they're 683 // much larger, so the cache-footprint-to-utility ratio 684 // of binaries is much lower for binaries. 685 // Not caching the link step also makes sure that repeated "go run" at least 686 // always rerun the linker, so that they don't get too fast. 687 // (We don't want people thinking go is a scripting language.) 688 // Note also that if we start caching binaries, then we will 689 // copy the binaries out of the cache to run them, and then 690 // that will mean the go process is itself writing a binary 691 // and then executing it, so we will need to defend against 692 // ETXTBSY problems as discussed in exec.go and golang.org/issue/22220. 693 if a.Mode == "build" { 694 r, err := os.Open(target) 695 if err == nil { 696 if a.output == nil { 697 panic("internal error: a.output not set") 698 } 699 outputID, _, err := c.Put(a.actionID, r) 700 r.Close() 701 if err == nil && cfg.BuildX { 702 sh.ShowCmd("", "%s # internal", joinUnambiguously(str.StringList("cp", target, c.OutputFile(outputID)))) 703 } 704 if b.NeedExport { 705 if err != nil { 706 return err 707 } 708 a.Package.Export = c.OutputFile(outputID) 709 a.Package.BuildID = a.buildID 710 } 711 } 712 } 713 714 return nil 715 } 716