debug/dwarf: Fail on reading incorrect offset due to skipped compilat…

…ion unit

src/debug/dwarf/entry.go

@@ -541,6 +541,8 @@ func (b *buf) entry(cu *Entry, atab abbrevTable, ubase Offset, vers int) *Entry
 			} else if a.tag == TagCompileUnit {
 				delay = append(delay, delayed{i, off, formAddrx})
 				break
+			} else {
+				b.error("Can't adjust offset: compilation unit skipped")
 			}
 
 			var err error
@@ -692,6 +694,8 @@ func (b *buf) entry(cu *Entry, atab abbrevTable, ubase Offset, vers int) *Entry
 			} else if a.tag == TagCompileUnit {
 				delay = append(delay, delayed{i, off, formStrx})
 				break
+			} else {
+				b.error("Can't adjust offset: compilation unit skipped")
 			}
 
 			val = resolveStrx(uint64(strBase), off)
@@ -752,6 +756,8 @@ func (b *buf) entry(cu *Entry, atab abbrevTable, ubase Offset, vers int) *Entry
 			} else if a.tag == TagCompileUnit {
 				delay = append(delay, delayed{i, off, formRnglistx})
 				break
+			} else {
+				b.error("Can't adjust offset: compilation unit skipped")
 			}
 
 			val = resolveRnglistx(uint64(rnglistsBase), off)
@@ -828,7 +834,7 @@ func (r *Reader) ByteOrder() binary.ByteOrder {
 
 // Seek positions the [Reader] at offset off in the encoded entry stream.
 // Offset 0 can be used to denote the first entry.
-func (r *Reader) Seek(off Offset) {
+func (r *Reader) seekImpl(off Offset) {
 	d := r.d
 	r.err = nil
 	r.lastChildren = false
@@ -856,6 +862,15 @@ func (r *Reader) Seek(off Offset) {
 	r.b = makeBuf(r.d, u, "info", off, u.data[off-u.off:])
 }
 
+// Seek positions the [Reader] at offset off in the encoded entry stream, and additionally sets the current
+// CU for the reader.
+func (r *Reader) Seek(off Offset) {
+	cuIdx := r.d.offsetToUnit(off)
+	r.seekImpl(r.d.unit[cuIdx].off)
+	r.Next()
+	r.seekImpl(off)
+}
+
 // maybeNextUnit advances to the next unit if this one is finished.
 func (r *Reader) maybeNextUnit() {
 	for len(r.b.data) == 0 && r.unit+1 < len(r.d.unit) {