Add sfdupes info: show the database's details (closes #117)

info prints the database's path and source, the sizes of the database
file and its -wal file, when the database file last changed, the schema
version and the record counts, one per line as a name, a tab and the
value. It opens the database as report does and shows an open and a
count phase on stderr. The schema version and the counts come from one
read-only transaction, the counts from one query.

seedDatabase now closes test databases out of WAL mode, as scan does;
otherwise info's own connection creates an empty -wal file. The open
phase's name and text became constants for the linter, and the scan
command moved to newScanCommand to keep newRootCommand short enough.

Judgement call: the lines are written after the last display ends, so they skip the display's stdout writer.

Model: opus-5-5

main_test.go

@@ -284,10 +284,16 @@ func assertFatalOutput(t *testing.T, stderr, stdout string) {
 }
 
 func TestRunMissingDatabaseIsFatal(t *testing.T) {
-	// README §Database: report and trees need an existing database; a
-	// missing one exits 1 with a message telling the user to run scan,
-	// after the line naming the database.
-	for _, name := range []string{cmdReport, cmdTrees} {
+	// README §Database: report, trees and info need an existing
+	// database; a missing one exits 1 with a message telling the user to
+	// run scan, after the line naming the database.
+	openLines := map[string]string{
+		cmdReport: reportOpenLine,
+		cmdTrees:  treesOpenLine,
+		cmdInfo:   infoOpenLine,
+	}
+
+	for name, openLine := range openLines {
 		t.Run(name, func(t *testing.T) {
 			path := testDBPath(t)
 			t.Setenv(databaseEnv, path)
@@ -305,13 +311,8 @@ func TestRunMissingDatabaseIsFatal(t *testing.T) {
 
 			// Each shows its open phase, as it starts and as it ends,
 			// before the error.
-			opening := reportOpenLine + "\n" + reportOpenLine + "\n"
-			if name == cmdTrees {
-				opening = treesOpenLine + "\n" + treesOpenLine + "\n"
-			}
-
 			want := name + ": database " + path + " (from " + databaseEnv +
-				")\n" + opening +
+				")\n" + openLine + "\n" + openLine + "\n" +
 				"sfdupes: " + path + ": no database (run \"sfdupes " +
 				"scan\" first, or set " + databaseEnv + ")\n"
 			if got := stderr(); got != want {
@@ -512,9 +513,9 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 	// exist at the default path on the machine running the test, so
 	// nothing here may create or read one: scan gets an operand that
 	// does not exist, which stops it before it takes the lock, and
-	// report and trees a cancelled context, which stops them before
-	// they read anything. report and trees show their open phase after
-	// the line.
+	// report, trees and info a cancelled context, which stops them
+	// before they read anything. They show their open phase after the
+	// line.
 	t.Setenv(databaseEnv, "")
 
 	// Whether scan calls the database new depends on that machine too.
@@ -552,6 +553,13 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 			want: "trees: database " + defaultDatabasePath + " (default)\n" +
 				treesOpenLine + "\n" + treesOpenLine + "\n",
 		},
+		cmdInfo: {
+			run: func() error {
+				return runInfo(cancelledContext(t), io.Discard)
+			},
+			want: "info: database " + defaultDatabasePath + " (default)\n" +
+				infoOpenLine + "\n" + infoOpenLine + "\n",
+		},
 	}
 
 	for name, tc := range cases {
@@ -578,6 +586,7 @@ func TestRunUsageErrors(t *testing.T) {
 		"scan without paths": {[]string{cmdScan}, usageMarker},
 		"report with args":   {[]string{cmdReport, "x"}, usageMarker},
 		"trees with args":    {[]string{cmdTrees, "x"}, usageMarker},
+		"info with args":     {[]string{cmdInfo, "x"}, usageMarker},
 		"unknown flag":       {[]string{cmdScan, "--nope", "/"}, usageMarker},
 		"unknown subcommand": {[]string{"nope"}, "unknown command"},
 	}
@@ -1258,7 +1267,7 @@ func TestRunReportsDuringScan(t *testing.T) {
 func TestRunStdoutClosedIsFatal(t *testing.T) {
 	// README §Error handling: a stdout write failure exits 1, reported
 	// in one line on stderr.
-	for _, name := range []string{cmdReport, cmdTrees} {
+	for _, name := range []string{cmdReport, cmdTrees, cmdInfo} {
 		t.Run(name, func(t *testing.T) {
 			t.Setenv(databaseEnv, testDBPath(t))
 
@@ -1308,6 +1317,7 @@ func TestStdoutWriteErrorPropagates(t *testing.T) {
 	cases := map[string]func(context.Context, io.Writer) error{
 		cmdReport: runReport,
 		cmdTrees:  runTrees,
+		cmdInfo:   runInfo,
 	}
 
 	for name, fn := range cases {