internal/pkg: deduplicate unwrapped input errors

This avoids repeating errors belonging to multiple instances of the same artifact in different locations in memory.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg.go

@@ -15,6 +15,7 @@ import (
 	"hash"
 	"io"
 	"io/fs"
+	"iter"
 	"maps"
 	"math"
 	"os"
@@ -1723,8 +1724,9 @@ retry:
 // An InputError describes inputs of a [FloodArtifact] which had failed to cure.
 type InputError map[Artifact]error
 
-// Error returns a user-facing, deterministic text representation of e.
-func (e InputError) Error() string {
+// unwrap returns an iterator over sorted, deduplicated [Artifact] and their
+// corresponding identifier.
+func (e InputError) unwrap() iter.Seq2[Artifact, unique.Handle[ID]] {
 	ir := NewIR()
 
 	type input struct {
@@ -1741,20 +1743,37 @@ func (e InputError) Error() string {
 		identBuf[0], identBuf[1] = a.id.Value(), b.id.Value()
 		return slices.Compare(identBuf[0][:], identBuf[1][:])
 	})
+	p = slices.CompactFunc(p, func(a, b input) bool { return a.id == b.id })
 
+	return func(yield func(Artifact, unique.Handle[ID]) bool) {
+		for _, i := range p {
+			if !yield(i.a, i.id) {
+				return
+			}
+		}
+	}
+}
+
+// Error returns a user-facing, deterministic text representation of e.
+func (e InputError) Error() string {
 	var buf strings.Builder
 	buf.WriteString("errors curing inputs:")
-	for _, i := range p {
-		buf.WriteString("\n\t" +
-			reportName(i.a, i.id) + ": " +
-			e[i.a].Error())
+	for a, id := range e.unwrap() {
+		buf.WriteString("\n\t")
+		buf.WriteString(reportName(a, id))
+		buf.WriteString(": ")
+		buf.WriteString(e[a].Error())
 	}
 	return buf.String()
 }
 
-// Unwrap returns a slice of underlying errors in unspecified order.
+// Unwrap returns a slice of underlying errors sorted by identifier.
 func (e InputError) Unwrap() []error {
-	return slices.AppendSeq(make([]error, 0, len(e)), maps.Values(e))
+	errs := make([]error, 0, len(e))
+	for a := range e.unwrap() {
+		errs = append(errs, e[a])
+	}
+	return errs
 }
 
 // enterCure must be called before entering an [Artifact] implementation.