graph/encoding/dot: ensure that generated DOT is syntactically valid

.travis.yml

@@ -34,6 +34,11 @@ before_install:
  # Required for dot parser checks.
  - ./.travis/install-gocc.sh 0e2cfc030005b281b2e5a2de04fa7fe1d5063722
 
+addons:
+  apt:
+    packages:
+     - graphviz
+
 go_import_path: gonum.org/v1/gonum
 
 # Get deps, build, test, and ensure the code is gofmt'ed.

graph/encoding/dot/encode_test.go

@@ -5,6 +5,8 @@
 package dot
 
 import (
+	"bytes"
+	"os/exec"
 	"testing"
 
 	"gonum.org/v1/gonum/graph"
@@ -1474,6 +1476,7 @@ func TestEncode(t *testing.T) {
 		if string(got) != test.want {
 			t.Errorf("unexpected DOT result for test %d:\ngot: %s\nwant:%s", i, got, test.want)
 		}
+		checkDOT(t, got)
 	}
 }
 
@@ -1729,5 +1732,24 @@ func TestEncodeMulti(t *testing.T) {
 		if string(got) != test.want {
 			t.Errorf("unexpected DOT result for test %d:\ngot: %s\nwant:%s", i, got, test.want)
 		}
+		checkDOT(t, got)
+	}
+}
+
+// checkDOT hands b to the dot executable if it exists and fails t if dot
+// returns an error.
+func checkDOT(t *testing.T, b []byte) {
+	dot, err := exec.LookPath("dot")
+	if err != nil {
+		t.Logf("skipping DOT syntax check: %v", err)
+		return
+	}
+	cmd := exec.Command(dot)
+	cmd.Stdin = bytes.NewReader(b)
+	stderr := &bytes.Buffer{}
+	cmd.Stderr = stderr
+	err = cmd.Run()
+	if err != nil {
+		t.Errorf("invalid DOT syntax: %v\n%s\ninput:\n%s", err, stderr.String(), b)
 	}
 }

graph/encoding/graphql/decode_test.go

@@ -5,8 +5,10 @@
 package graphql
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
+	"os/exec"
 	"sort"
 	"strconv"
 	"strings"
@@ -147,6 +149,7 @@ func TestDecode(t *testing.T) {
 		if gotDOT != test.wantDOT {
 			t.Errorf("unexpected DOT encoding for %q:\ngot:\n%s\nwant:\n%s", test.name, gotDOT, test.wantDOT)
 		}
+		checkDOT(t, b)
 	}
 }
 
@@ -219,3 +222,21 @@ func (a attributes) Attributes() []encoding.Attribute {
 	}
 	return attr
 }
+
+// checkDOT hands b to the dot executable if it exists and fails t if dot
+// returns an error.
+func checkDOT(t *testing.T, b []byte) {
+	dot, err := exec.LookPath("dot")
+	if err != nil {
+		t.Logf("skipping DOT syntax check: %v", err)
+		return
+	}
+	cmd := exec.Command(dot)
+	cmd.Stdin = bytes.NewReader(b)
+	stderr := &bytes.Buffer{}
+	cmd.Stderr = stderr
+	err = cmd.Run()
+	if err != nil {
+		t.Errorf("invalid DOT syntax: %v\n%s\ninput:\n%s", err, stderr.String(), b)
+	}
+}