fix file-based filtering symbolization

If file-filtered logging is enabled, V looks up its caller's source
file path using runtime.Callers and runtime.FuncForPC. However,
runtime.Callers returns return PCs, which aren't necessarily in the
same source file as the call site. For example, if F calls V, and F's
call to V is immediately followed by an inlined called to another
function, say G, then symbolizing V's return PC will actually return
G's file and line number, not F's.

Fix this by subtracting 1 from the PC returned by runtime.Callers to
back up from the return PC to the call PC.

An arguably better fix would be to use runtime.CallersFrames, which
abstracts away such details. Unfortunately, CallersFrames allocates
(at least as of Go 1.15), so we avoid it here.

klog.go

@@ -1328,9 +1328,14 @@ func V(level Level) Verbose {
 		if runtime.Callers(2, logging.pcs[:]) == 0 {
 			return newVerbose(level, false)
 		}
-		v, ok := logging.vmap[logging.pcs[0]]
+		// runtime.Callers returns "return PCs", but we want
+		// to look up the symbolic information for the call,
+		// so subtract 1 from the PC. runtime.CallersFrames
+		// would be cleaner, but allocates.
+		pc := logging.pcs[0] - 1
+		v, ok := logging.vmap[pc]
 		if !ok {
-			v = logging.setV(logging.pcs[0])
+			v = logging.setV(pc)
 		}
 		return newVerbose(level, v >= level)
 	}