1 files changed, 92 insertions, 13 deletions

diff --git a/internal/pkg/pkg.go b/internal/pkg/pkg.go
index 78e62e85..aadfa308 100644
--- a/internal/pkg/pkg.go
+++ b/internal/pkg/pkg.go
@@ -368,7 +368,7 @@ var _ error = InvalidLookupError{}
 
 // GetArtifact returns the identifier pathname and checksum of an [Artifact].
 // Calling Pathname with an [Artifact] not part of the slice returned by
-// [Artifact.Dependencies] panics.
+// [Artifact.Inputs] panics.
 func (f *FContext) GetArtifact(a Artifact) (
 	pathname *check.Absolute,
 	checksum unique.Handle[Checksum],
@@ -678,6 +678,10 @@ const (
 
 	// CIgnoreSubstitutes disables content-based dependency substitution.
 	CIgnoreSubstitutes
+
+	// CExternShallow arranges for only non-flood inputs to be fetched when
+	// curing an [Artifact] available via the external cache.
+	CExternShallow
 )
 
 // toplevel holds [context.WithCancel] over caller-supplied context, where all
@@ -1914,25 +1918,57 @@ func (c *Cache) tryExtern(ctx context.Context, id unique.Handle[ID]) (
 }
 
 // cureMany concurrently collects outcome of multiple [Artifact].
-func (c *Cache) cureMany(inputs []Artifact, r map[Artifact]cureRes) error {
+func (c *Cache) cureMany(
+	inputs []Artifact,
+	r map[Artifact]cureRes,
+	shallow bool,
+) ([]bool, error) {
 	var wg sync.WaitGroup
 	wg.Add(len(inputs))
+	var mask []bool
 	res := make([]cureRes, len(inputs))
 	errs := make(DependencyCureError, 0, len(inputs))
 	var errsMu sync.Mutex
+	if shallow {
+		mask = make([]bool, len(inputs))
+	}
 	for i, d := range inputs {
+		if shallow {
+			if _, ok := d.(FloodArtifact); ok {
+				mask[i] = true
+				wg.Done()
+				continue
+			}
+
+			if kc, ok := d.(KnownChecksum); ok {
+				res[i].checksum = unique.Make(kc.Checksum())
+				wg.Done()
+				continue
+			}
+		}
 		pending := pendingArtifactDep{d, &res[i], &errs, &errsMu, &wg}
 		go pending.cure(c)
 	}
 	wg.Wait()
 
 	if len(errs) > 0 {
-		return &errs
+		return mask, &errs
 	}
 	for i, p := range res {
+		if shallow && mask[i] {
+			continue
+		}
 		r[inputs[i]] = p
 	}
-	return nil
+	return mask, nil
+}
+
+// HangingInputError describes an input of an [Artifact] on a sparse cache
+// without an outcome available locally or via [External].
+type HangingInputError unique.Handle[ID]
+
+func (e HangingInputError) Error() string {
+	return Encode(unique.Handle[ID](e).Value()) + " is unavailable"
 }
 
 // cure implements Cure without acquiring a read lock on abortMu. cure must not
@@ -2193,12 +2229,62 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 		break
 
 	case FloodArtifact:
+		var externChecksum unique.Handle[Checksum]
+		if externChecksum, err = c.tryExtern(ctx, id); err != nil {
+			if c.msg.IsVerbose() {
+				c.msg.Verbosef("extern %s: %v", reportName(ca, id), err)
+			}
+			return
+		}
+		extern := externChecksum != zeroChecksum
+		shallow := extern && c.flags&CExternShallow != 0
+
 		inputs := a.Inputs()
 		f := FContext{t, make(map[Artifact]cureRes, len(inputs))}
-		if err = c.cureMany(inputs, f.inputs); err != nil {
+		var mask []bool
+		if mask, err = c.cureMany(inputs, f.inputs, shallow); err != nil {
 			return
 		}
 
+		if shallow {
+			for i, d := range inputs {
+				if !mask[i] {
+					continue
+				}
+
+				if kc, ok := d.(KnownChecksum); ok {
+					f.inputs[d] = cureRes{checksum: unique.Make(kc.Checksum())}
+					continue
+				}
+
+				var sum unique.Handle[Checksum]
+				did := c.Ident(d)
+				sum, err = c.tryLocal(did)
+				if err != nil {
+					if !errors.Is(err, os.ErrNotExist) {
+						return
+					}
+
+					sum, err = c.tryExtern(ctx, did)
+					if err != nil {
+						return
+					}
+					if sum == zeroChecksum {
+						if c.msg.IsVerbose() {
+							c.msg.Verbosef(
+								"input %s not available",
+								reportName(d, did),
+							)
+						}
+						err = HangingInputError(did)
+						return
+					}
+				}
+
+				f.inputs[d] = cureRes{checksum: sum}
+			}
+		}
+
 		sh := sha512.New384()
 		err = c.encode(sh, a, f.inputs)
 		if err != nil {
@@ -2245,14 +2331,7 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 
 		defer f.destroy(&err)
 
-		var externChecksum unique.Handle[Checksum]
-		if externChecksum, err = c.tryExtern(ctx, id); err != nil {
-			if c.msg.IsVerbose() {
-				c.msg.Verbosef("extern %s: %v", reportName(ca, id), err)
-			}
-			return
-		}
-		if externChecksum != zeroChecksum {
+		if extern {
 			if checksum != zeroChecksum && externChecksum != checksum {
 				err = &ChecksumMismatchError{externChecksum.Value(), checksum.Value()}
 				if c.msg.IsVerbose() {