cmd/go: add errors obtaining c compiler version to cache key

If there's an error getting the version of the c compiler, add the error
to the input used to produce the cache key. In the case where we can't
parse the version, the text of the output of the command is part of the
error, so different unparseable versions will produce different cache
keys. Before, we wouldn't add anything to the key when there was an
error getting the version, so we wouldn't distinguish a missing compiler
from one where we couldn't parse the version.

Fixes #64589

Change-Id: I27f853e8ff40002e2f045b2210633b38f93d0130
Reviewed-on: https://go-review.googlesource.com/c/go/+/648196
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

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

@@ -301,12 +301,16 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 		// compiler changes we rebuild the package.
 		if ccID, _, err := b.gccToolID(ccExe[0], "c"); err == nil {
 			fmt.Fprintf(h, "CC ID=%q\n", ccID)
+		} else {
+			fmt.Fprintf(h, "CC ID ERROR=%q\n", err)
 		}
 		if len(p.CXXFiles)+len(p.SwigCXXFiles) > 0 {
 			cxxExe := b.cxxExe()
 			fmt.Fprintf(h, "CXX=%q %q\n", cxxExe, cxxflags)
 			if cxxID, _, err := b.gccToolID(cxxExe[0], "c++"); err == nil {
 				fmt.Fprintf(h, "CXX ID=%q\n", cxxID)
+			} else {
+				fmt.Fprintf(h, "CXX ID ERROR=%q\n", err)
 			}
 		}
 		if len(p.FFiles) > 0 {
@@ -314,6 +318,8 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 			fmt.Fprintf(h, "FC=%q %q\n", fcExe, fflags)
 			if fcID, _, err := b.gccToolID(fcExe[0], "f95"); err == nil {
 				fmt.Fprintf(h, "FC ID=%q\n", fcID)
+			} else {
+				fmt.Fprintf(h, "FC ID ERROR=%q\n", err)
 			}
 		}
 		// TODO(rsc): Should we include the SWIG version?

src/cmd/go/testdata/script/build_cc_cache_issue64589.txt

@@ -0,0 +1,100 @@
+# Regression test for https://go.dev/issue/64589:
+# This test is very similar to build_cc_cache_issue64423. Issue #64423
+# was that we weren't properly parsing the versions output by the compiler.
+# That test checked that we could parse the version and incorporate the
+# version into the hash for the action id. This issue #64589 is that
+# we treat all errors getting the version of the compiler the same, so
+# we'd get the same action id for a missing compiler vs one whose
+# version is unparseable. So the test now first does a run with a compiler
+# that produces unparseable version output, and then runs it again with a missing
+# compiler and ensures the command doesn't return the cached output for the
+# first run when running the second run.
+
+[!cgo] skip
+[short] skip 'builds and links a fake clang binary'
+[!cc:clang] skip 'test is specific to clang version parsing'
+
+# Save the location of the real clang command for our fake one to use.
+go run ./which clang
+cp stdout $WORK/.realclang
+
+# Build a fake clang and ensure that it is the one in $PATH.
+mkdir $WORK/bin
+go build -o $WORK/bin/clang$GOEXE ./fakeclang
+[!GOOS:plan9] env PATH=$WORK${/}bin
+[GOOS:plan9] env path=$WORK${/}bin
+
+# Force CGO_ENABLED=1 so that the following commands should error
+# out if the fake clang doesn't work.
+env CGO_ENABLED=1
+
+# The bug in https://go.dev/issue/64589 resulted in cache keys that
+# didn't contain any information about the error getting the compiler version.
+# Since the bug was in cache key computation, isolate the cache:
+# if we change the way caching works, we want the test to fail
+# instead of accidentally reusing the cached information from a
+# previous test run.
+env GOCACHE=$WORK${/}.cache
+mkdir $GOCACHE
+
+go build -x runtime/cgo
+
+	# Tell our fake clang to stop working.
+	# Previously, 'go build -x runtime/cgo' would continue to
+	# succeed because both the broken clang and the non-broken one
+	# resulted in a cache key with no clang version information.
+env GO_BREAK_CLANG=1
+! go build -x runtime/cgo
+stderr '# runtime/cgo\nGO_BREAK_CLANG is set'
+
+-- go.mod --
+module example/issue64589
+go 1.20
+-- which/main.go --
+package main
+
+import (
+	"os"
+	"os/exec"
+)
+
+func main() {
+	path, err := exec.LookPath(os.Args[1])
+	if err != nil {
+		panic(err)
+	}
+	os.Stdout.WriteString(path)
+}
+-- fakeclang/main.go --
+package main
+
+import (
+	"bytes"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"slices"
+)
+
+func main() {
+	if os.Getenv("GO_BREAK_CLANG") != "" {
+		os.Stderr.WriteString("GO_BREAK_CLANG is set\n")
+		os.Exit(1)
+	}
+
+	b, err := os.ReadFile(filepath.Join(os.Getenv("WORK"), ".realclang"))
+	if err != nil {
+		log.Fatal(err)
+	}
+	if slices.Contains(os.Args, "-###") { // We are being run by gccToolID to determine the tool id used in the action id.
+		return // The important thing is that we don't print the string "version"!
+	}
+	clang := string(bytes.TrimSpace(b))
+	cmd := exec.Command(clang, os.Args[1:]...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		log.Fatal(err)
+	}
+}