cmd/jsonimports: add static analyzer for consistent "json" imports (#17669)

This migrates an internal tool to open source
so that we can run it on the tailscale.com module as well.
We add the "util/safediff" also as a dependency of the tool.

This PR does not yet set up a CI to run this analyzer.

Updates tailscale/corp#791

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/safediff/diff_test.go

@@ -0,0 +1,196 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package safediff
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func init() { diffTest = true }
+
+func TestLines(t *testing.T) {
+	// The diffs shown below technically depend on the stability of cmp,
+	// but that should be fine for sufficiently simple diffs like these.
+	// If the output does change, that would suggest a significant regression
+	// in the optimality of cmp's diffing algorithm.
+
+	x := `{
+	"firstName": "John",
+	"lastName": "Smith",
+	"isAlive": true,
+	"age": 27,
+	"address": {
+		"streetAddress": "21 2nd Street",
+		"city": "New York",
+		"state": "NY",
+		"postalCode": "10021-3100"
+	},
+	"phoneNumbers": [{
+		"type": "home",
+		"number": "212 555-1234"
+	}, {
+		"type": "office",
+		"number": "646 555-4567"
+	}],
+	"children": [
+		"Catherine",
+		"Thomas",
+		"Trevor"
+	],
+	"spouse": null
+}`
+	y := x
+	y = strings.ReplaceAll(y, `"New York"`, `"Los Angeles"`)
+	y = strings.ReplaceAll(y, `"NY"`, `"CA"`)
+	y = strings.ReplaceAll(y, `"646 555-4567"`, `"315 252-8888"`)
+
+	wantDiff := `
+… 5 identical lines
+  	"address": {
+  		"streetAddress": "21 2nd Street",
+- 		"city": "New York",
+- 		"state": "NY",
++ 		"city": "Los Angeles",
++ 		"state": "CA",
+  		"postalCode": "10021-3100"
+  	},
+… 3 identical lines
+  	}, {
+  		"type": "office",
+- 		"number": "646 555-4567"
++ 		"number": "315 252-8888"
+  	}],
+… 7 identical lines
+`[1:]
+	gotDiff, gotTrunc := Lines(x, y, -1)
+	if d := cmp.Diff(gotDiff, wantDiff); d != "" {
+		t.Errorf("Lines mismatch (-got +want):\n%s\ngot:\n%s\nwant:\n%s", d, gotDiff, wantDiff)
+	} else if gotTrunc == true {
+		t.Errorf("Lines: output unexpectedly truncated")
+	}
+
+	wantDiff = `
+… 5 identical lines
+  	"address": {
+  		"streetAddress": "21 2nd Street",
+- 		"city": "New York",
+- 		"state": "NY",
++ 		"city": "Los Angeles",
+… 15 identical, 1 removed, and 2 inserted lines
+`[1:]
+	gotDiff, gotTrunc = Lines(x, y, 200)
+	if d := cmp.Diff(gotDiff, wantDiff); d != "" {
+		t.Errorf("Lines mismatch (-got +want):\n%s\ngot:\n%s\nwant:\n%s", d, gotDiff, wantDiff)
+	} else if gotTrunc == false {
+		t.Errorf("Lines: output unexpectedly not truncated")
+	}
+
+	wantDiff = "… 17 identical, 3 removed, and 3 inserted lines\n"
+	gotDiff, gotTrunc = Lines(x, y, 0)
+	if d := cmp.Diff(gotDiff, wantDiff); d != "" {
+		t.Errorf("Lines mismatch (-got +want):\n%s\ngot:\n%s\nwant:\n%s", d, gotDiff, wantDiff)
+	} else if gotTrunc == false {
+		t.Errorf("Lines: output unexpectedly not truncated")
+	}
+
+	x = `{
+	"unrelated": [
+		"unrelated",
+	],
+	"related": {
+		"unrelated": [
+			"unrelated",
+		],
+		"related": {
+			"unrelated": [
+				"unrelated",
+			],
+			"related": {
+				"related": "changed",
+			},
+			"unrelated": [
+				"unrelated",
+			],
+		},
+		"unrelated": [
+			"unrelated",
+		],
+	},
+	"unrelated": [
+		"unrelated",
+	],
+}`
+	y = strings.ReplaceAll(x, "changed", "CHANGED")
+
+	wantDiff = `
+… 4 identical lines
+  	"related": {
+… 3 identical lines
+  		"related": {
+… 3 identical lines
+  			"related": {
+- 				"related": "changed",
++ 				"related": "CHANGED",
+  			},
+… 3 identical lines
+  		},
+… 3 identical lines
+  	},
+… 4 identical lines
+`[1:]
+	gotDiff, gotTrunc = Lines(x, y, -1)
+	if d := cmp.Diff(gotDiff, wantDiff); d != "" {
+		t.Errorf("Lines mismatch (-got +want):\n%s\ngot:\n%s\nwant:\n%s", d, gotDiff, wantDiff)
+	} else if gotTrunc == true {
+		t.Errorf("Lines: output unexpectedly truncated")
+	}
+
+	x = `{
+	"ACLs": [
+		{
+			"Action": "accept",
+			"Users":  ["group:all"],
+			"Ports":  ["tag:tmemes:80"],
+		},
+	],
+}`
+	y = strings.ReplaceAll(x, "tag:tmemes:80", "tag:tmemes:80,8383")
+	wantDiff = `
+  {
+  	"ACLs": [
+  		{
+  			"Action": "accept",
+  			"Users":  ["group:all"],
+- 			"Ports":  ["tag:tmemes:80"],
++ 			"Ports":  ["tag:tmemes:80,8383"],
+  		},
+  	],
+  }
+`[1:]
+	gotDiff, gotTrunc = Lines(x, y, -1)
+	if d := cmp.Diff(gotDiff, wantDiff); d != "" {
+		t.Errorf("Lines mismatch (-got +want):\n%s\ngot:\n%s\nwant:\n%s", d, gotDiff, wantDiff)
+	} else if gotTrunc == true {
+		t.Errorf("Lines: output unexpectedly truncated")
+	}
+}
+
+func FuzzDiff(f *testing.F) {
+	f.Fuzz(func(t *testing.T, x, y string, maxSize int) {
+		const maxInput = 1e3
+		if len(x) > maxInput {
+			x = x[:maxInput]
+		}
+		if len(y) > maxInput {
+			y = y[:maxInput]
+		}
+		diff, _ := Lines(x, y, maxSize) // make sure this does not panic
+		if strings.Count(diff, "\n") > 1 && maxSize >= 0 && len(diff) > maxSize {
+			t.Fatal("maxSize exceeded")
+		}
+	})
+}