internal/profile: actually return errors in postDecode

As spotted by staticcheck, the body did keep track of errors by sharing
a single err variable, but its last value was never used as the function
simply finished by returning nil.

To prevent postDecode from erroring on empty profiles,
which breaks TestEmptyProfile, add a check at the top of the function.

Update the runtime/pprof test accordingly,
since the default units didn't make sense for an empty profile anyway.

Change-Id: I188cd8337434adf9169651ab5c914731b8b20f39
Reviewed-on: https://go-review.googlesource.com/c/go/+/483137
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: mvdan

src/internal/profile/encode.go

@@ -207,6 +207,9 @@ 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)
@@ -291,7 +294,7 @@ func (p *Profile) postDecode() error {
 	p.commentX = nil
 	p.DefaultSampleType, err = getString(p.stringTable, &p.defaultSampleTypeX, err)
 	p.stringTable = nil
-	return nil
+	return err
 }
 
 func (p *ValueType) decoder() []decoder {

src/runtime/pprof/proto_test.go

@@ -64,13 +64,9 @@ func TestConvertCPUProfileEmpty(t *testing.T) {
 	}
 
 	// Expected PeriodType and SampleType.
-	periodType := &profile.ValueType{Type: "cpu", Unit: "nanoseconds"}
-	sampleType := []*profile.ValueType{
-		{Type: "samples", Unit: "count"},
-		{Type: "cpu", Unit: "nanoseconds"},
-	}
+	sampleType := []*profile.ValueType{{}, {}}
 
-	checkProfile(t, p, 2000*1000, periodType, sampleType, nil, "")
+	checkProfile(t, p, 2000*1000, nil, sampleType, nil, "")
 }
 
 func f1() { f1() }