add Lineunit.Err method

.github/workflows/memci.yml

@@ -17,11 +17,17 @@ jobs:
 
       - uses: actions/setup-go@v6
         with:
-          go-version: "1.24"
+          # TinyGo 0.42 builds with Go 1.25 through 1.27 and refuses anything
+          # outside that window, so the two versions below move together.
+          go-version: "1.26"
+      - uses: acifani/setup-tinygo@v2
+        with:
+          tinygo-version: "0.42.0"
 
       - uses: soypat/memci@main
         with:
           args: -targets ./cmd/... -kind package
           # bindiff lives in this repo, so measure with the copy under test
           # rather than installing a published one.
           bindiff: go run ./cmd/bindiff
+          tinygo: tinygo

build/elfutil/dwarf_test.go

@@ -131,8 +131,7 @@ func TestStreamedLineTableMatchesResident(t *testing.T) {
 	sec := xdwarf.Sections{Str: bytes.NewReader(str), LineStr: bytes.NewReader(lineStr)}
 
 	// The fixture's largest unit header wants 1867 bytes of aux, so 2048 is
-	// about as tight as this walk goes -- and tight is the point, since a small
-	// aux means a refill across nearly every read.
+	// about as tight as this walk goes.
 	for _, aux := range []int{2048, 4096, 16384} {
 		t.Run("aux="+itoa(aux), func(t *testing.T) {
 			sec.Line = bytes.NewReader(raw)
@@ -181,10 +180,9 @@ func TestStreamBufferTooSmall(t *testing.T) {
 		var next int64
 		next, err = xdwarf.DecodeLineUnit(&u, sec, off, size, auxBuf)
 		if err == nil {
-			// LineUnit.Rows reports no error of its own, so a rewind that
-			// happens mid-program is only visible on the next unit's decode.
 			for range u.Rows {
 			}
+			err = u.Err()
 		}
 		if err != nil {
 			break

build/xdwarf/line.go

@@ -102,9 +102,12 @@ type LineUnit struct {
 
 	sec Sections
 	hdr []byte // The unit header, at the front of aux. Inline strRefs index it.
-	win []byte // The tail of aux, the fill buffer VisitRows streams through.
+	win []byte // The tail of aux, the fill buffer Rows streams through.
 	// The opcode stream, as absolute offsets within .debug_line.
 	progStart, progEnd int64
+	// err holds why the last [LineUnit.Rows] walk stopped early. An iterator
+	// cannot return one, so it is kept here for [LineUnit.Err].
+	err error
 }
 
 // Row is one row of the line table matrix: the state of the line program at a
@@ -316,7 +319,7 @@ func (u *LineUnit) appendStr(dst []byte, r strRef) ([]byte, error) {
 // appendCStr appends the NUL-terminated string at off to dst, reading straight
 // onto dst's own spare capacity so it needs no scratch of its own. That matters
 // because the aux tail, the obvious place for scratch, is simultaneously the
-// window a [LineUnit.VisitRows] walk is streaming the opcode program through.
+// window a [LineUnit.Rows] walk is streaming the opcode program through.
 func appendCStr(dst []byte, r io.ReaderAt, off int64) ([]byte, error) {
 	if r == nil {
 		return dst, errNoStrSection
@@ -348,7 +351,7 @@ func appendCStr(dst []byte, r io.ReaderAt, off int64) ([]byte, error) {
 // aux is scratch owned by the caller and borrowed by dst until the next decode
 // into the same buffer. The unit's header is copied to its front, where dst's
 // directory and file tables point, and its tail becomes the fill buffer
-// [LineUnit.VisitRows] streams the opcode program through. An aux too small to
+// [LineUnit.Rows] streams the opcode program through. An aux too small to
 // hold both reports how large it needs to be. Passing the same dst and aux back
 // on the next call makes decoding allocation-free.
 func DecodeLineUnit(dst *LineUnit, sec Sections, off, size int64, aux []byte) (next int64, err error) {
@@ -723,8 +726,19 @@ func (u *LineUnit) skipForm(c *cursor, f Form) error {
 	return c.err
 }
 
-// Rows is an [iter.Seq] iterator implementation oveer LineUnit rows.
+// Err reports why the last [LineUnit.Rows] walk stopped, or nil if it ran to
+// the end of the opcode program. A malformed unit, or a lookback window too
+// small for the decoder to step back through, ends a walk early; because an
+// [iter.Seq] cannot return an error, a caller that needs to tell a truncated
+// walk from a complete one has to ask here.
+//
+// [LineUnit.Rows] clears it, so Err describes the most recent walk only.
+func (u *LineUnit) Err() error { return u.err }
+
+// Rows is an [iter.Seq] iterator over the unit's line table rows. A walk that
+// ends early leaves its reason in [LineUnit.Err].
 func (u *LineUnit) Rows(yield func(r Row) bool) {
+	u.err = nil
 	var c streamCursor
 	c.config(u.sec.Line, u.win, u.progStart, u.progEnd, u.sec.byteOrder())
 
@@ -828,6 +842,9 @@ func (u *LineUnit) Rows(yield func(r Row) bool) {
 			}
 		}
 	}
+	// Only reached by running the program out; every early return above is a
+	// caller stopping the walk, which leaves u.err nil as cleared on entry.
+	u.err = c.err
 }
 
 func (u *LineUnit) resetRow() Row {

build/xdwarf/line_test.go

@@ -275,6 +275,39 @@ func TestRowsStop(t *testing.T) {
 	if n != 3 {
 		t.Errorf("visited %d rows after stopping at 3", n)
 	}
+	// Stopping is the caller's own doing. Reporting it as an error would make
+	// every early exit look like a malformed unit.
+	if err := u.Err(); err != nil {
+		t.Errorf("stopping the walk reported an error: %s", err)
+	}
+}
+
+// TestRowsErr pins the rest of the Err contract: a walk that reaches the end of
+// the opcode program reports nothing, and each walk describes only itself.
+func TestRowsErr(t *testing.T) {
+	sec, size := loadSections(t, "../../testdata/helloc.elf")
+	var u xdwarf.LineUnit
+	if _, err := xdwarf.DecodeLineUnit(&u, sec, 0, size, make([]byte, defaultAux)); err != nil {
+		t.Fatal(err)
+	}
+	rows := 0
+	for range u.Rows {
+		rows++
+	}
+	if rows == 0 {
+		t.Fatal("the unit yielded no rows")
+	}
+	if err := u.Err(); err != nil {
+		t.Fatalf("a complete walk reported an error: %s", err)
+	}
+	// A second walk over the same unit must start from a clean slate rather
+	// than inheriting the first walk's verdict.
+	for range u.Rows {
+		break
+	}
+	if err := u.Err(); err != nil {
+		t.Errorf("a second walk inherited an error: %s", err)
+	}
 }
 
 func TestDecodeLineUnitRejectsBadInput(t *testing.T) {