[dev.link] cmd/internal/goobj2: add index fingerprint to object file

The new object files use indices for symbol references, instead
of names. Fundamental to the design, it requires that the
importing and imported packages have consistent view of symbol
indices. The Go command should already ensure this, when using
"go build". But in case it goes wrong, it could lead to obscure
errors like run-time crashes. It would be better to check the
index consistency at build time.

To do that, we add a fingerprint to each object file, which is
a hash of symbol indices. In the object file it records the
fingerprints of all imported packages, as well as its own
fingerprint. At link time, the linker checks that a package's
fingerprint matches the fingerprint recorded in the importing
packages, and issue an error if they don't match.

This CL does the first part: introducing the fingerprint in the
object file, and propagating fingerprints through
importing/exporting by the compiler. It is not yet used by the
linker. Next CL will do.

Change-Id: I0aa372da652e4afb11f2867cb71689a3e3f9966e
Reviewed-on: https://go-review.googlesource.com/c/go/+/229617
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index 2b0c45d6b2..6d7f42ed0b 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -98,8 +98,9 @@ func WriteObjFile(ctxt *Link, bout *bio.Writer, pkgpath string) {
 	w.wr.WriteByte(1)
 
 	// Autolib
-	for _, pkg := range ctxt.Imports {
-		w.writeString(pkg)
+	for _, p := range ctxt.Imports {
+		w.writeString(p.Pkg)
+		// This object format ignores p.Fingerprint.
 	}
 	w.writeString("")