internal/restic: Return summary from BlobSet.String

Fixes #4449: error messages from prune were too long to fit in scroll
buffers.

internal/restic/blob_set.go

@@ -1,6 +1,10 @@
 package restic
 
-import "sort"
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
 
 // BlobSet is a set of blobs.
 type BlobSet map[BlobHandle]struct{}
@@ -103,11 +107,27 @@ func (s BlobSet) List() BlobHandles {
 	return list
 }
 
+// String produces a human-readable representation of ids.
+// It is meant for producing error messages,
+// so it only returns a summary if ids is long.
 func (s BlobSet) String() string {
-	str := s.List().String()
-	if len(str) < 2 {
-		return "{}"
-	}
+	const maxelems = 10
 
-	return "{" + str[1:len(str)-1] + "}"
+	sb := new(strings.Builder)
+	sb.WriteByte('{')
+	n := 0
+	for k := range s {
+		if n != 0 {
+			sb.WriteByte(' ')
+		}
+		sb.WriteString(k.String())
+
+		if n++; n == maxelems {
+			fmt.Fprintf(sb, " (%d more)", len(s)-n-1)
+			break
+		}
+	}
+	sb.WriteByte('}')
+
+	return sb.String()
 }

internal/restic/blob_set_test.go

@@ -0,0 +1,32 @@
+package restic
+
+import (
+	"math/rand"
+	"regexp"
+	"testing"
+
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestBlobSetString(t *testing.T) {
+	s := NewBlobSet()
+
+	rtest.Equals(t, "{}", s.String())
+
+	id, _ := ParseID(
+		"1111111111111111111111111111111111111111111111111111111111111111")
+	s.Insert(BlobHandle{ID: id, Type: TreeBlob})
+	rtest.Equals(t, "{<tree/11111111>}", s.String())
+
+	var h BlobHandle
+	for i := 0; i < 100; i++ {
+		h.Type = DataBlob
+		_, _ = rand.Read(h.ID[:])
+		s.Insert(h)
+	}
+
+	r := regexp.MustCompile(
+		`^{(?:<(?:data|tree)/[0-9a-f]{8}> ){10}\(90 more\)}$`)
+	str := s.String()
+	rtest.Assert(t, r.MatchString(str), "%q doesn't match pattern", str)
+}