internal/pkg: optionally suppress via assumed checksum

This is quite error-prone and causes cache inconsistency similar to the store inconsistency seen on nix when a similar condition happens. Keep this behind a flag in case it is ever beneficial.

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

internal/pkg/pkg.go

@@ -507,6 +507,20 @@ const (
 	// CSchedIdle arranges for the [ext.SCHED_IDLE] scheduling priority to be
 	// set for [KindExec] and [KindExecNet] containers.
 	CSchedIdle
+
+	// CAssumeChecksum enables the use of [KnownChecksum] for duplicate function
+	// call suppression via the on-disk cache.
+	//
+	// This may cause incorrect cure outcome if an impossible checksum is
+	// specified that matches an output already present in the on-disk cache.
+	// This may be avoided by purposefully specifying a statistically
+	// unattainable checksum, like the zero value.
+	//
+	// While this optimisation might seem appealing, it is almost never
+	// applicable in real world use. Almost every time this path was taken, it
+	// was caused by an incorrect checksum accidentally left behind while
+	// bumping a package. Only enable this if you are really sure you need it.
+	CAssumeChecksum
 )
 
 // Cache is a support layer that implementations of [Artifact] can use to store
@@ -1041,7 +1055,7 @@ func (c *Cache) finaliseIdent(
 // [FileArtifact] to the filesystem. If err is nil, the caller is responsible
 // for closing the resulting [io.ReadCloser].
 func (c *Cache) openFile(f FileArtifact) (r io.ReadCloser, err error) {
-	if kc, ok := f.(KnownChecksum); ok {
+	if kc, ok := f.(KnownChecksum); c.flags&CAssumeChecksum != 0 && ok {
 		c.checksumMu.RLock()
 		r, err = os.Open(c.base.Append(
 			dirChecksum,
@@ -1484,16 +1498,18 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 			checksums,
 		)
 
-		c.checksumMu.RLock()
-		checksumFi, err = os.Stat(checksumPathname.String())
-		c.checksumMu.RUnlock()
+		if c.flags&CAssumeChecksum != 0 {
+			c.checksumMu.RLock()
+			checksumFi, err = os.Stat(checksumPathname.String())
+			c.checksumMu.RUnlock()
 
-		if err != nil {
-			if !errors.Is(err, os.ErrNotExist) {
-				return
+			if err != nil {
+				if !errors.Is(err, os.ErrNotExist) {
+					return
+				}
+
+				checksumFi, err = nil, nil
 			}
-
-			checksumFi, err = nil, nil
 		}
 	}