Absorb: ask for the passphrase once the archive starts arriving

With gpg --decrypt x.tar.gpg | vault absorb, vault printed its prompt
while gpg was still writing to the terminal, which buried it, so the
command looked hung. Peek at standard input first: the upstream program
has finished with the terminal by the time data arrives, and an empty
pipe now fails without asking at all.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/guide/vault-guide.tex

@@ -581,7 +581,11 @@ vault absorb old < archive.tar.gz
 
 Each file is encrypted straight from the stream, so nothing is unpacked to
 disk first. Directories need no entry of their own; links, devices and any
-name that tries to climb out with \file{..} are skipped with a note. If the
+name that tries to climb out with \file{..} are skipped with a note.
+vault asks for your passphrase only once the archive starts arriving, so
+when it comes out of something that asks first, like
+\cmd{gpg --decrypt secrets.tar.gpg | vault absorb}, gpg's prompt comes
+and goes before vault's appears. If the
 archive stops halfway through a file, that file isn't stored and vault
 stops; everything before it is already safely in.
 

docs/vault-absorb.1

@@ -35,8 +35,12 @@ or
 is dropped.
 .PP
 The passphrase prompt reads from the terminal, not standard input, so
-it still works with an archive piped in. The vault is locked exclusively
-for the whole run.
+it still works with an archive piped in. It waits until the archive starts
+arriving, so a program feeding the pipe, such as
+.BR gpg (1)
+asking for its own passphrase, has finished with the terminal first.
+An empty pipe fails without asking at all.
+The vault is locked exclusively for the whole run.
 .SH OPTIONS
 .TP
 .BI \-\-key " PATH"

main.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"bufio"
 	"errors"
 	"flag"
 	"fmt"
@@ -158,10 +159,17 @@ func main() {
 		if len(pos) == 1 {
 			dest = strings.Trim(pos[0], "/")
 		}
+		// Wait for the archive to start arriving before asking for the
+		// passphrase, so a program feeding the pipe (gpg, ssh) has finished
+		// with the terminal and cannot bury the prompt.
+		in := bufio.NewReader(os.Stdin)
+		if _, err := in.Peek(1); err != nil {
+			fail("nothing to absorb on standard input")
+		}
 		s := open(*keyPath, *vaultDir, true)
 		cat, err := s.catalogue()
 		check(err)
-		check(absorb(s, cat, os.Stdin, dest))
+		check(absorb(s, cat, in, dest))
 	case "list", "ls":
 		want(0, 1)
 		prefix := ""