gopls/internal/lsp/filecache: silently discard CRC errors on macOS

On macOS we observe builder panics caused by CRC failures reading
filecache entries: the checksum is all zero but the rest is fine.

The underlying cause is still a mystery:
lockedfile.Write(name, io.MultiReader(len, val, checksum))
results in three write system calls to the locked *os.File,
so while it is natural that we might observe fewer than three
portions of the file if the process is interrupted, it
doesn't explain why we might see the third portion existing
with the wrong value.  I suspect a macOS file system bug.

Fixes golang/go#59895

Change-Id: I43d0765b37d695cce0919425f6a919623da1ee46
Reviewed-on: https://go-review.googlesource.com/c/tools/+/492035
Run-TryBot: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
gopls-CI: kokoro <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: adonovan

gopls/internal/lsp/filecache/filecache.go

@@ -33,6 +33,7 @@ import (
 	"log"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"sync"
 	"sync/atomic"
@@ -81,7 +82,16 @@ func Get(kind string, key [32]byte) ([]byte, error) {
 	// issue #59289. TODO(adonovan): stop printing the entire file
 	// once we've seen enough reports to understand the pattern.
 	if binary.LittleEndian.Uint32(checksum) != crc32.ChecksumIEEE(value) {
-		return nil, bug.Errorf("internal error in filecache.Get(%q, %x): invalid checksum at end of %d-byte file %s:\n%q",
+		// Darwin has repeatedly displayed a problem (#59895)
+		// whereby the checksum portion (and only it) is zero,
+		// which suggests a bug in its file system . Don't
+		// panic, but keep an eye on other failures for now.
+		errorf := bug.Errorf
+		if binary.LittleEndian.Uint32(checksum) == 0 && runtime.GOOS == "darwin" {
+			errorf = fmt.Errorf
+		}
+
+		return nil, errorf("internal error in filecache.Get(%q, %x): invalid checksum at end of %d-byte file %s:\n%q",
 			kind, key, len(data), name, data)
 	}