gopls/internal/lsp/filecache: use TempDir if UserCacheDir fails us

On the builders, HOME=/ which causes UserCacheDir to be non-writable.
This change attempts to MkdirAll the UserCacheDir and, if that fails,
we fail back to TempDir, which is assumed to exist and be writable.

Updates golang/go#57638

Change-Id: I6eb81c59b50f90a62c103a2511425fa2f24452fa
Reviewed-on: https://go-review.googlesource.com/c/tools/+/460917
Reviewed-by: Bryan Mills <[email protected]>
gopls-CI: kokoro <[email protected]>
Run-TryBot: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

gopls/internal/lsp/filecache/filecache.go

@@ -180,6 +180,20 @@ func getCacheDir() string {
 		}
 		goplsDir := filepath.Join(userDir, "gopls")
 
+		// UserCacheDir may return a nonexistent directory
+		// (in which case we must create it, which may fail),
+		// or it may return a non-writable directory, in
+		// which case we should ideally respect the user's express
+		// wishes (e.g. XDG_CACHE_HOME) and not write somewhere else.
+		// Sadly UserCacheDir doesn't currently let us distinguish
+		// such intent from accidental misconfiguraton such as HOME=/
+		// in a CI builder. So, we check whether the gopls subdirectory
+		// can be created (or already exists) and not fall back to /tmp.
+		// See also https://github.com/golang/go/issues/57638.
+		if os.MkdirAll(goplsDir, 0700) != nil {
+			goplsDir = filepath.Join(os.TempDir(), "gopls")
+		}
+
 		// Start the garbage collector.
 		go gc(goplsDir)