internal/profile: fully decode proto even if there are no samples

This is a partial revert of CL 483137.

CL 483137 started checking errors in postDecode, which is good. Now we
can catch more malformed pprof protos. However this made
TestEmptyProfile fail, so an early return was added when the profile was
"empty" (no samples).

Unfortunately, this was problematic. Profiles with no samples can still
be valid, but skipping postDecode meant that the resulting Profile was
missing values from the string table. In particular, net/http/pprof
needs to parse empty profiles in order to pass through the sample and
period types to a final output proto. CL 483137 broke this behavior.

internal/profile.Parse is only used in two places: in cmd/compile to
parse PGO pprof profiles, and in net/http/pprof to parse before/after
pprof profiles for delta profiles. In both cases, the input is never
literally empty (0 bytes). Even a pprof proto with no samples still
contains some header fields, such as sample and period type. Upstream
github.com/google/pprof/profile even has an explicit error on 0 byte
input, so `go tool pprof` will not support such an input.

Thus TestEmptyProfile was misleading; this profile doesn't need to
support empty input at all.

Resolve this by removing TestEmptyProfile and replacing it with an
explicit error on empty input, as upstream
github.com/google/pprof/profile has. For non-empty input, always run
postDecode to ensure the string table is processed.

TestConvertCPUProfileEmpty is reverted back to assert the values from
before CL 483137. Note that in this case "Empty" means no samples, not a
0 byte input.

Continue to allow empty files for PGO in order to minimize the chance of
last minute breakage if some users have empty files.

Fixes #64566.

Change-Id: I83a1f0200ae225ac6da0009d4b2431fe215b283f
Reviewed-on: https://go-review.googlesource.com/c/go/+/547996
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

3 files changed, 13 insertions, 22 deletions

diff --git a/src/internal/profile/encode.go b/src/internal/profile/encode.go
index 72d6fe2fa7..94d04bf094 100644
--- a/src/internal/profile/encode.go
+++ b/src/internal/profile/encode.go
@@ -207,9 +207,6 @@ var profileDecoder = []decoder{
 // suffix X) and populates the corresponding exported fields.
 // The unexported fields are cleared up to facilitate testing.
 func (p *Profile) postDecode() error {
-	if p.Empty() {
-		return nil
-	}
 	var err error
 
 	mappings := make(map[uint64]*Mapping)
diff --git a/src/internal/profile/profile.go b/src/internal/profile/profile.go
index c779bb2b11..02d1bed3de 100644
--- a/src/internal/profile/profile.go
+++ b/src/internal/profile/profile.go
@@ -141,10 +141,14 @@ func Parse(r io.Reader) (*Profile, error) {
 		}
 		orig = data
 	}
-	if p, err = parseUncompressed(orig); err != nil {
-		if p, err = parseLegacy(orig); err != nil {
-			return nil, fmt.Errorf("parsing profile: %v", err)
-		}
+
+	var lErr error
+	p, pErr := parseUncompressed(orig)
+	if pErr != nil {
+		p, lErr = parseLegacy(orig)
+	}
+	if pErr != nil && lErr != nil {
+		return nil, fmt.Errorf("parsing profile: not a valid proto profile (%w) or legacy profile (%w)", pErr, lErr)
 	}
 
 	if err := p.CheckValid(); err != nil {
@@ -155,6 +159,7 @@ func Parse(r io.Reader) (*Profile, error) {
 
 var errUnrecognized = fmt.Errorf("unrecognized profile format")
 var errMalformed = fmt.Errorf("malformed profile format")
+var ErrNoData = fmt.Errorf("empty input file")
 
 func parseLegacy(data []byte) (*Profile, error) {
 	parsers := []func([]byte) (*Profile, error){
@@ -180,6 +185,10 @@ func parseLegacy(data []byte) (*Profile, error) {
 }
 
 func parseUncompressed(data []byte) (*Profile, error) {
+	if len(data) == 0 {
+		return nil, ErrNoData
+	}
+
 	p := &Profile{}
 	if err := unmarshal(data, p); err != nil {
 		return nil, err
diff --git a/src/internal/profile/profile_test.go b/src/internal/profile/profile_test.go
index e1963f3351..84158b6233 100644
--- a/src/internal/profile/profile_test.go
+++ b/src/internal/profile/profile_test.go
@@ -5,24 +5,9 @@
 package profile
 
 import (
-	"bytes"
 	"testing"
 )
 
-func TestEmptyProfile(t *testing.T) {
-	var buf bytes.Buffer
-	p, err := Parse(&buf)
-	if err != nil {
-		t.Error("Want no error, got", err)
-	}
-	if p == nil {
-		t.Fatal("Want a valid profile, got <nil>")
-	}
-	if !p.Empty() {
-		t.Errorf("Profile should be empty, got %#v", p)
-	}
-}
-
 func TestParseContention(t *testing.T) {
 	tests := []struct {
 		name    string