Allow binary override for profiles with no mappings (#89)

The go runtime generates profiles in profile.proto format
without symbols or mappings. The expectation is that these
can be symbolized by passing the binary name to pprof.

The mechanism pprof uses relies to override the binary relies
on there being a mapping, and previously we moved the creation
of fake mappings to the legacy profile handlers, so profiles
parsed from profile.proto with no mappings can no longer be
symbolized.

Special case this situation to create a fake mapping and associate
all locations to it if there is a command line override but no
mappings.

internal/driver/fetch.go

 		// Use $HOME/pprof/binaries as default directory for local symbolization binaries
 		searchPath = filepath.Join(os.Getenv("HOME"), "pprof", "binaries")
 	}
-
 mapping:
-	for i, m := range p.Mapping {
+	for _, m := range p.Mapping {
 		var baseName string
-		// Replace executable filename/buildID with the overrides from source.
-		// Assumes the executable is the first Mapping entry.
-		if i == 0 {
-			if s.ExecName != "" {
-				m.File = s.ExecName
-			}
-			if s.BuildID != "" {
-				m.BuildID = s.BuildID
-			}
-		}
 		if m.File != "" {
 			baseName = filepath.Base(m.File)
 		}
 			}
 		}
 	}
+	// Replace executable filename/buildID with the overrides from source.
+	// Assumes the executable is the first Mapping entry.
+	if execName, buildId := s.ExecName, s.BuildID; execName != "" || buildId != "" {
+		if len(p.Mapping) == 0 {
+			// If there are no mappings, add a fake mapping to attempt symbolization.
+			// This is useful for some profiles generated by the golang runtime, which
+			// do not include any mappings. Symbolization with a fake mapping will only
+			// be successful against a non-PIE binary.
+			m := &profile.Mapping{ID: 1}
+			p.Mapping = []*profile.Mapping{m}
+			for _, l := range p.Location {
+				l.Mapping = m
+			}
+		}
+		m := p.Mapping[0]
+		if execName != "" {
+			m.File = execName
+		}
+		if buildId != "" {
+			m.BuildID = buildId
+		}
+	}
 }
 
 // fetch fetches a profile from source, within the timeout specified,

internal/driver/fetch_test.go

 	// Intercept http.Get calls from HTTPFetcher.
 	httpGet = stubHTTPGet
 
-	for _, source := range [][2]string{
-		{path + "go.crc32.cpu", "go.crc32.cpu"},
-		{"http://localhost/profile?file=cppbench.cpu", "cppbench.cpu"},
+	type testcase struct {
+		source, execName string
+	}
+
+	for _, tc := range []testcase{
+		{path + "go.crc32.cpu", ""},
+		{path + "go.nomappings.crash", "/bin/gotest.exe"},
+		{"http://localhost/profile?file=cppbench.cpu", ""},
 	} {
-		p, _, err := fetch(source[0], 0, 10*time.Second, &proftest.TestUI{t, 0})
+		p, _, _, err := grabProfile(&source{ExecName: tc.execName}, tc.source, 0, nil, testObj{}, &proftest.TestUI{t, 0})
 		if err != nil {
-			t.Fatalf("%s: %s", source[0], err)
+			t.Fatalf("%s: %s", tc.source, err)
 		}
 		if len(p.Sample) == 0 {
-			t.Errorf("want non-zero samples")
+			t.Errorf("%s: want non-zero samples", tc.source)
+		}
+		if e := tc.execName; e != "" {
+			switch {
+			case len(p.Mapping) == 0 || p.Mapping[0] == nil:
+				t.Errorf("%s: want mapping[0].execName == %s, got no mappings", tc.source, e)
+			case p.Mapping[0].File != e:
+				t.Errorf("%s: want mapping[0].execName == %s, got %s", tc.source, e, p.Mapping[0].File)
+			}
 		}
 	}
 }