cmd/go: avoid recording GOROOT_FINAL in precompiled C archives

C archives for packages in GOROOT are shipped along with binary
releases of the Go toolchain. Although we build the toolchain with
GOROOT_FINAL set, we don't know actually know where the release will
be installed: the user's real GOROOT can differ arbitrarily from our
GOROOT_FINAL.

(In the specific case of toolchains installed through golang.org/dl
wrappers, the release's GOROOT_FINAL is /usr/local/go but the actual
GOROOT to which the release is installed is
$HOME/sdk/$(go env GOVERSION).)

Fixes #50183
Updates #48319

Change-Id: If10a42f90c725300bbcb89c3b5b01a2d93ab6ef7
Reviewed-on: https://go-review.googlesource.com/c/go/+/380915
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/work/exec.go

@@ -236,11 +236,13 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 		}
 	} else if p.Goroot {
 		// The Go compiler always hides the exact value of $GOROOT
-		// when building things in GOROOT, but the C compiler
-		// merely rewrites GOROOT to GOROOT_FINAL.
-		if len(p.CFiles) > 0 {
-			fmt.Fprintf(h, "goroot %s\n", cfg.GOROOT_FINAL)
-		}
+		// when building things in GOROOT.
+		//
+		// The C compiler does not, but for packages in GOROOT we rewrite the path
+		// as though -trimpath were set, so that we don't invalidate the build cache
+		// (and especially any precompiled C archive files) when changing
+		// GOROOT_FINAL. (See https://go.dev/issue/50183.)
+		//
 		// b.WorkDir is always either trimmed or rewritten to
 		// the literal string "/tmp/go-build".
 	} else if !strings.HasPrefix(p.Dir, b.WorkDir) {
@@ -2337,7 +2339,7 @@ func (b *Builder) ccompile(a *Action, p *load.Package, outfile string, flags []s
 	// directives pointing to the source directory. It should not generate those
 	// when -trimpath is enabled.
 	if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
-		if cfg.BuildTrimpath {
+		if cfg.BuildTrimpath || p.Goroot {
 			// Keep in sync with Action.trimpath.
 			// The trimmed paths are a little different, but we need to trim in the
 			// same situations.
@@ -2359,8 +2361,6 @@ func (b *Builder) ccompile(a *Action, p *load.Package, outfile string, flags []s
 				to = filepath.Join("/_", toPath)
 			}
 			flags = append(flags[:len(flags):len(flags)], "-fdebug-prefix-map="+from+"="+to)
-		} else if p.Goroot && cfg.GOROOT_FINAL != cfg.GOROOT {
-			flags = append(flags[:len(flags):len(flags)], "-fdebug-prefix-map="+cfg.GOROOT+"="+cfg.GOROOT_FINAL)
 		}
 	}
 

src/cmd/go/testdata/script/cgo_stale_precompiled.txt

@@ -0,0 +1,17 @@
+# Regression test for https://go.dev/issue/47215 and https://go.dev/issue/50183:
+# A mismatched $GOROOT_FINAL or missing $CC caused the C dependencies of the net
+# package to appear stale, and it could not be rebuilt due to a missing $CC.
+
+[!cgo] skip
+
+# Control case: net must not already be stale.
+! stale net
+
+# https://go.dev/issue/47215: a missing $(go env CC) caused the precompiled net to be stale.
+[!plan9] env PATH=''  # Guaranteed not to include $(go env CC)!
+[plan9] env path=''
+! stale net  # issue #47215
+
+# https://go.dev/issue/50183: a mismatched GOROOT_FINAL caused net to be stale.
+env GOROOT_FINAL=$WORK${/}goroot
+! stale net