Add sfdupes dwim: scan / then print duplicate trees (closes #154)

dwim runs what "sfdupes scan /" runs with no flags, then what
"sfdupes trees" runs, and ends with a stderr line saying that
"sfdupes trees" prints the report again. An interrupt or fatal scan
error ends it before the report, as scan ends.

Model: opus-5-5

README.md

@@ -256,7 +256,7 @@ sfdupes scan [--workers N] [-x] [--no-default-excludes] PATH...
 sfdupes report > dupes.tsv
 sfdupes trees > dupetrees.tsv
 sfdupes info
-sfdupes dwim
+sfdupes dwim > dupetrees.tsv
 sfdupes --version
 sfdupes [command] --help
 ```
@@ -828,8 +828,7 @@ even after the terminal is gone.
 `SIGINT` or `SIGTERM` stops the scan as it stops `scan` (see "Stopping a scan").
 `dwim` then prints no report and exits 1. A fatal scan error also ends it before
 the report, with the same message and exit code as `scan`. Warnings for skipped
-files do not stop it. `dwim` stops catching signals when the scan ends, so a
-signal during the report ends it at once, as it ends `trees`.
+files do not stop it.
 
 ### `info` mode
 
@@ -887,8 +886,6 @@ scan: new database /var/lib/sfdupes/db.sqlite (default)
 trees: database /srv/tmp/sfdupes/db.db (from SFDUPES_DATABASE)
 ```
 
-`dwim` prints the `scan` line first, and the `trees` line after the scan.
-
 A relative path is printed with the working directory in front of it. The
 working directory is printed as `pwd -P` prints it, with its symlinks resolved.
 The rest of the path is printed as set, so a `..` in it stays. The line names
@@ -1172,8 +1169,8 @@ showing the count it stopped at.
 ### Error handling and exit codes
 
 - `0`: success, even when files were skipped with warnings.
-- `1`: a fatal error, a `scan` stopped by `SIGINT` or `SIGTERM`, or a `dwim`
-  stopped by either during its scan. A fatal error is one of these:
+- `1`: a fatal error, or a `scan` or `dwim` stopped by `SIGINT` or `SIGTERM`. A
+  fatal error is one of these:
     - an operand that does not exist, or whose real path `scan` cannot find;
     - a list of mounted filesystems that `scan` cannot read;
     - another `scan` holding the lock (see "The scan lock");

dwim.go

@@ -11,16 +11,10 @@ import (
 // runDwim implements the dwim subcommand: it runs what "sfdupes scan
 // root" runs with no flags, then what "sfdupes trees" runs, and ends
 // with a line on stderr saying how to print the report again. The dwim
-// command passes "/" as root. It catches SIGINT and SIGTERM as scan
-// does, but only during the scan. An error from the scan, an interrupt
-// included, ends it before the report. During the report, a signal
-// ends it at once, as it ends trees.
+// command passes "/" as root. An error from the scan, an interrupt
+// included, ends it before the report.
 func runDwim(ctx context.Context, root string, stdout io.Writer) error {
-	scanCtx, stop := interruptContext(ctx)
-	err := runScan(scanCtx, []string{root}, runtime.NumCPU(), false, false)
-
-	stop()
-
+	err := runScan(ctx, []string{root}, runtime.NumCPU(), false, false)
 	if err != nil {
 		return err
 	}

dwim_test.go

@@ -16,12 +16,12 @@ func TestRunDwimPrintsLargestTreesFirst(t *testing.T) {
 	root := realTempDir(t)
 
 	small := []string{
-		filepath.Dir(writeFile(t, root, "a1/f.bin", pattern(1, 300))),
-		filepath.Dir(writeFile(t, root, "a2/f.bin", pattern(1, 300))),
+		filepath.Dir(writeFile(t, root, "small1/f.bin", pattern(1, 300))),
+		filepath.Dir(writeFile(t, root, "small2/f.bin", pattern(1, 300))),
 	}
 	large := []string{
-		filepath.Dir(writeFile(t, root, "b1/f.bin", pattern(2, 5000))),
-		filepath.Dir(writeFile(t, root, "b2/f.bin", pattern(2, 5000))),
+		filepath.Dir(writeFile(t, root, "large1/f.bin", pattern(2, 5000))),
+		filepath.Dir(writeFile(t, root, "large2/f.bin", pattern(2, 5000))),
 	}
 
 	var stdout bytes.Buffer

main.go

@@ -14,7 +14,7 @@
 //	sfdupes report > dupes.tsv
 //	sfdupes trees > dupetrees.tsv
 //	sfdupes info
-//	sfdupes dwim
+//	sfdupes dwim > dupetrees.tsv
 //	sfdupes --version
 //
 // See README.md for the complete specification.
@@ -208,6 +208,9 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 		Short: "Scan / with the default excludes, then print the duplicate-tree report",
 		Args:  cobra.NoArgs,
 		RunE: runE(func(ctx context.Context, _ []string) error {
+			ctx, stop := interruptContext(ctx)
+			defer stop()
+
 			return runDwim(ctx, "/", stdout)
 		}),
 	}