Add tests for CheckValid, fix a couple crashers. (#248)

Add tests for CheckValid, fix a couple crashers.

profile/profile.go

 		return fmt.Errorf("missing sample type information")
 	}
 	for _, s := range p.Sample {
+		if s == nil {
+			return fmt.Errorf("profile has nil sample")
+		}
 		if len(s.Value) != sampleLen {
-			return fmt.Errorf("mismatch: sample has: %d values vs. %d types", len(s.Value), len(p.SampleType))
+			return fmt.Errorf("mismatch: sample has %d values vs. %d types", len(s.Value), len(p.SampleType))
 		}
 		for _, l := range s.Location {
 			if l == nil {
 	// Check that there are no duplicate ids
 	mappings := make(map[uint64]*Mapping, len(p.Mapping))
 	for _, m := range p.Mapping {
+		if m == nil {
+			return fmt.Errorf("profile has nil mapping")
+		}
 		if m.ID == 0 {
 			return fmt.Errorf("found mapping with reserved ID=0")
 		}
 	}
 	functions := make(map[uint64]*Function, len(p.Function))
 	for _, f := range p.Function {
+		if f == nil {
+			return fmt.Errorf("profile has nil function")
+		}
 		if f.ID == 0 {
 			return fmt.Errorf("found function with reserved ID=0")
 		}
 	}
 	locations := make(map[uint64]*Location, len(p.Location))
 	for _, l := range p.Location {
+		if l == nil {
+			return fmt.Errorf("profile has nil location")
+		}
 		if l.ID == 0 {
 			return fmt.Errorf("found location with reserved id=0")
 		}

profile/profile_test.go

 }
 
 func TestParseError(t *testing.T) {
-
 	testcases := []string{
 		"",
 		"garbage text",
 	}
 }
 
+func TestCheckValid(t *testing.T) {
+	const path = "testdata/java.cpu"
+
+	inbytes, err := ioutil.ReadFile(path)
+	if err != nil {
+		t.Fatalf("failed to read profile file %q: %v", path, err)
+	}
+	p, err := Parse(bytes.NewBuffer(inbytes))
+	if err != nil {
+		t.Fatalf("failed to parse profile %q: %s", path, err)
+	}
+
+	for _, tc := range []struct {
+		mutateFn func(*Profile)
+		wantErr  string
+	}{
+		{
+			mutateFn: func(p *Profile) { p.SampleType = nil },
+			wantErr:  "missing sample type information",
+		},
+		{
+			mutateFn: func(p *Profile) { p.Sample[0] = nil },
+			wantErr:  "profile has nil sample",
+		},
+		{
+			mutateFn: func(p *Profile) { p.Sample[0].Value = append(p.Sample[0].Value, 0) },
+			wantErr:  "sample has 3 values vs. 2 types",
+		},
+		{
+			mutateFn: func(p *Profile) { p.Sample[0].Location[0] = nil },
+			wantErr:  "sample has nil location",
+		},
+		{
+			mutateFn: func(p *Profile) { p.Location[0] = nil },
+			wantErr:  "profile has nil location",
+		},
+		{
+			mutateFn: func(p *Profile) { p.Mapping = append(p.Mapping, nil) },
+			wantErr:  "profile has nil mapping",
+		},
+		{
+			mutateFn: func(p *Profile) { p.Function[0] = nil },
+			wantErr:  "profile has nil function",
+		},
+	} {
+		t.Run(tc.wantErr, func(t *testing.T) {
+			p := p.Copy()
+			tc.mutateFn(p)
+			if err := p.CheckValid(); err == nil {
+				t.Errorf("CheckValid(): got no error, want error %q", tc.wantErr)
+			} else if !strings.Contains(err.Error(), tc.wantErr) {
+				t.Errorf("CheckValid(): got error %v, want error %q", err, tc.wantErr)
+			}
+		})
+	}
+}
+
 // leaveTempfile leaves |b| in a temporary file on disk and returns the
 // temp filename. This is useful to recover a profile when the test
 // fails.