cmd/go: don't lock .mod and .sum files for read in overlay

On Plan 9, locking a file requires a chmod call. In general, the go
command should not modify files in the overlay, even metadata. With
this change, we won't lock these files for reading.

The go command already reported errors when attempting to write these
files if they were in the overlay, but this change moves those checks
to the point of access for clearer error
messages. cmd/go/internal/lockedfile no longer imports
cmd/go/internal/fsys.

Fixes #44700

Change-Id: Ib544459dd6cf56ca0f7a27b3285f045f08040d7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/333012
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 17 insertions, 1 deletions

diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 09136b7de1..a8cbd9fe16 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -412,7 +412,16 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 	}
 
 	gomod := ModFilePath()
-	data, err := lockedfile.Read(gomod)
+	var data []byte
+	var err error
+	if gomodActual, ok := fsys.OverlayPath(gomod); ok {
+		// Don't lock go.mod if it's part of the overlay.
+		// On Plan 9, locking requires chmod, and we don't want to modify any file
+		// in the overlay. See #44700.
+		data, err = os.ReadFile(gomodActual)
+	} else {
+		data, err = lockedfile.Read(gomodActual)
+	}
 	if err != nil {
 		base.Fatalf("go: %v", err)
 	}
@@ -1026,6 +1035,13 @@ func commitRequirements(ctx context.Context, goVersion string, rs *Requirements)
 		}
 		return
 	}
+	gomod := ModFilePath()
+	if _, ok := fsys.OverlayPath(gomod); ok {
+		if dirty {
+			base.Fatalf("go: updates to go.mod needed, but go.mod is part of the overlay specified with -overlay")
+		}
+		return
+	}
 
 	new, err := modFile.Format()
 	if err != nil {