cmd/go/internal/modfetch/codehost: report git errors more accurately

Previously, if you attempted to fetch a private repository, or your
Git/curl client failed for an unknown reason, codehost would return an
UnknownRevisionError, which reported that a given revision in go.mod
was "unknown". This is confusing to many users who can go look in
their browser for example and see that the commit-ish exists.

Instead check whether "git ls-remote" exited with an error, and if so,
return that instead of the UnknownRevision message.

Fixes #42751.

Change-Id: I0dbded878b2818280e61126a4493767d719ad577
Reviewed-on: https://go-review.googlesource.com/c/go/+/297950
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

2 files changed, 36 insertions, 0 deletions

diff --git a/src/cmd/go/internal/modfetch/codehost/git.go b/src/cmd/go/internal/modfetch/codehost/git.go
index 72005e27d5..4d4964edf4 100644
--- a/src/cmd/go/internal/modfetch/codehost/git.go
+++ b/src/cmd/go/internal/modfetch/codehost/git.go
@@ -296,6 +296,9 @@ func (r *gitRepo) stat(rev string) (*RevInfo, error) {
 	// Or maybe it's the prefix of a hash of a named ref.
 	// Try to resolve to both a ref (git name) and full (40-hex-digit) commit hash.
 	r.refsOnce.Do(r.loadRefs)
+	// loadRefs may return an error if git fails, for example segfaults, or
+	// could not load a private repo, but defer checking to the else block
+	// below, in case we already have the rev in question in the local cache.
 	var ref, hash string
 	if r.refs["refs/tags/"+rev] != "" {
 		ref = "refs/tags/" + rev
@@ -332,6 +335,9 @@ func (r *gitRepo) stat(rev string) (*RevInfo, error) {
 			hash = rev
 		}
 	} else {
+		if r.refsErr != nil {
+			return nil, r.refsErr
+		}
 		return nil, &UnknownRevisionError{Rev: rev}
 	}
 
diff --git a/src/cmd/go/testdata/script/mod_get_private_vcs.txt b/src/cmd/go/testdata/script/mod_get_private_vcs.txt
index 514b0a7a53..8b01eac62c 100644
--- a/src/cmd/go/testdata/script/mod_get_private_vcs.txt
+++ b/src/cmd/go/testdata/script/mod_get_private_vcs.txt
@@ -9,3 +9,33 @@ env GOPROXY=direct
 stderr 'Confirm the import path was entered correctly.'
 stderr 'If this is a private repository, see https://golang.org/doc/faq#git_https for additional information.'
 ! stdout .
+
+# Fetching a nonexistent commit should return an "unknown revision"
+# error message.
+! go get github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b
+stderr '^go get: github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b: invalid version: unknown revision 86186f3aba07ed0212cfb944f3398997d2d07c6b$'
+! stdout .
+
+! go get github.com/golang/nonexist@master
+stderr '^Confirm the import path was entered correctly.$'
+stderr '^If this is a private repository, see https://golang.org/doc/faq#git_https for additional information.$'
+! stderr 'unknown revision'
+! stdout .
+
+[!linux] stop
+
+# Test that Git clone errors will be shown to the user instead of a generic
+# "unknown revision" error. To do this we want to force git ls-remote to return
+# an error we don't already have special handling for. See golang/go#42751.
+#
+# Set XDG_CONFIG_HOME to tell Git where to look for the git config file listed
+# below, which turns on ssh.
+env XDG_CONFIG_HOME=$TMPDIR
+! go install github.com/golang/nonexist@master
+stderr 'fatal: Could not read from remote repository.'
+! stderr 'unknown revision'
+! stdout .
+
+-- $TMPDIR/git/config --
+[url "git@github.com:"]
+    insteadOf = https://github.com/