A tar.gz cut short loses the share of its files asked for

CHANGELOG.md

@@ -36,6 +36,13 @@ because it turns other people's test suites red.
   5493 B, to 320x240 at 7732 B and to 640x480 at 18722 B, about a tenth above
   where it did. A file between the old step and the new one is not byte for
   byte what 0.5.0 wrote.
+- **A tar.gz 16 KiB to 64 KiB larger than the files inside it now opens in
+  Alpine Linux.** busybox, the `gzip` and `tar` of Alpine and many small
+  container images, refused these archives as corrupted data - the default
+  64 KiB one among them. The gzip header now carries at most 16 KiB of the
+  padding, which busybox reads, and the padding file inside the archive takes
+  the rest, as it did for larger archives. Those archives are not byte for
+  byte what 0.5.0 wrote. Every other tar.gz is unchanged.
 
 ### Added
 
@@ -78,8 +85,7 @@ because it turns other people's test suites red.
   the manifest's `damage` now carries `cut`: `in` (`content`, `padding`,
   `structure` or `end`) and `content_kept_bytes`, and `content_bytes` where
   the length of the content is known. A cut in the padding also gets the
-  note `cut_in_padding`. In a compressed tar.gz a cut in the padding is
-  reported as a cut in the content.
+  note `cut_in_padding`.
 - **`truncate` cuts the picture of a PNG, a GIF, a JPEG or an icon by the
   percentage you set.** `--damage truncate:missing=90` on a 1 MB PNG or GIF
   used to cut only padding and leave the whole picture, and on a JPEG or a
@@ -94,6 +100,13 @@ because it turns other people's test suites red.
   file inside. In a small file a cut of a few percent can fall in the index at
   its end, and then every file inside is kept - the manifest says `in: end`.
   Files nothing cuts are unchanged.
+- **`truncate` cuts the files inside a tar.gz by the percentage you set,
+  compressed or not.** `--damage truncate:missing=50` on a 1 MB tar.gz used
+  to cut only padding. Now it removes half of the files' content, so
+  `tar -xzf` gets back the first part of the file inside and then stops at
+  the unexpected end. When the cut is shorter than the end of the archive -
+  1% of 64 KiB - it falls there and every file is kept, and the manifest says
+  `in: end`. Files nothing cuts are unchanged.
 
 - **tfg is on npm, for tests in Node.**
   `npm install --save-dev testing-files-generator` gives Playwright, Vitest

internal/format/opc/cut.go

@@ -38,6 +38,10 @@ func SettleForACut(parts []Part, shape Shape, want, cutAt int64) (Package, error
 	}
 	pad := want - shape.Bare
 	front := format.PaddingInFront(0, c.content, cutAt, want)
+	// The parts are marked with their length whether or not a part goes in
+	// front: the cut stops the writing before the mark behind them, so a cut
+	// inside them says how much of them there was only from this.
+	p.Content = c.content
 	if front <= 0 || pad < c.frontOverhead {
 		return p, nil
 	}

internal/format/opc/opc.go

@@ -267,16 +267,16 @@ func write(ctx context.Context, w io.Writer, p Package, withPadding bool) error
 	// archive/zip writes it (archive.StreamMap). A package laid out for a cut
 	// opens with a padding part, and the content starts behind it.
 	behind := FillerName
+	content := p.Content
+	if content <= 0 {
+		content = -1
+	}
 	if p.Front == NoFiller {
-		format.Mark(w, format.RoleContent, 0, -1)
+		format.Mark(w, format.RoleContent, 0, content)
 	} else {
 		if err := writePaddingPart(ctx, zw, zm, paddingPart{FillerName, p.Seed, p.Front, withPadding}); err != nil {
 			return err
 		}
-		content := p.Content
-		if content <= 0 {
-			content = -1
-		}
 		format.Mark(w, format.RoleContent, zm.Pos(), content)
 		behind = FillerEndName
 	}

internal/format/targz/compress.go

@@ -31,8 +31,8 @@ import (
 //
 // Measured 2026-09-01 across levels 1, 6 and 9 and targets from 64 KB to
 // 10 MB: every one landed on the ordered size, and the remainder left for the
-// extra field came out between 567 and 3759 bytes - far inside the 65 531 the
-// field holds (O163).
+// extra field came out between 567 and 3759 bytes - far inside the 16 384 the
+// field holds (extraPaddingLimit).
 //
 // This costs passes, and the cost is real: one pass over a 10 MB archive is
 // 25 ms at level 1 and 140 ms at level 6, against 8 ms stored. It buys the one
@@ -143,7 +143,7 @@ func nextFiller(target, got, filler int64) (next, extra int64, useExtra, done bo
 		// compiler broke it. Probe: tools/probes/targzsettle.
 		//
 		// Undershooting is safe and overshooting is not: the extra field adds
-		// exactly what it is given, up to 65 531 B, so a round that lands under
+		// exactly what it is given, up to extraPaddingLimit, so a round that lands under
 		// the target finishes on the next pass. A whole block is well inside
 		// that.
 		next := filler - (2 - deficit) - tarBlock