internal/pkg: split off context common

For making these methods available to RContext.

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

internal/pkg/pkg.go

@@ -65,18 +65,23 @@ func MustDecode(s string) (checksum Checksum) {
 	return
 }
 
+// common holds elements and receives methods shared between different contexts.
+type common struct {
+	// Address of underlying [Cache], should be zeroed or made unusable after
+	// Cure returns and must not be exposed directly.
+	cache *Cache
+}
+
 // TContext is passed to [TrivialArtifact.Cure] and provides information and
 // methods required for curing the [TrivialArtifact].
 //
 // Methods of TContext are safe for concurrent use. TContext is valid
 // until [TrivialArtifact.Cure] returns.
 type TContext struct {
-	// Address of underlying [Cache], should be zeroed or made unusable after
-	// [TrivialArtifact.Cure] returns and must not be exposed directly.
-	cache *Cache
-
 	// Populated during [Cache.Cure].
 	work, temp *check.Absolute
+
+	common
 }
 
 // destroy destroys the temporary directory and joins its errors with the error
@@ -104,10 +109,10 @@ func (t *TContext) destroy(errP *error) {
 }
 
 // Unwrap returns the underlying [context.Context].
-func (t *TContext) Unwrap() context.Context { return t.cache.ctx }
+func (c *common) Unwrap() context.Context { return c.cache.ctx }
 
 // GetMessage returns [message.Msg] held by the underlying [Cache].
-func (t *TContext) GetMessage() message.Msg { return t.cache.msg }
+func (c *common) GetMessage() message.Msg { return c.cache.msg }
 
 // GetWorkDir returns a pathname to a directory which [Artifact] is expected to
 // write its output to. This is not the final resting place of the [Artifact]
@@ -126,13 +131,13 @@ func (t *TContext) GetTempDir() *check.Absolute { return t.temp }
 // If err is nil, the caller must close the resulting [io.ReadCloser] and return
 // its error, if any. Failure to read r to EOF may result in a spurious
 // [ChecksumMismatchError], or the underlying implementation may block on Close.
-func (t *TContext) Open(a Artifact) (r io.ReadCloser, err error) {
+func (c *common) Open(a Artifact) (r io.ReadCloser, err error) {
 	if f, ok := a.(FileArtifact); ok {
-		return t.cache.openFile(f)
+		return c.cache.openFile(f)
 	}
 
 	var pathname *check.Absolute
-	if pathname, _, err = t.cache.Cure(a); err != nil {
+	if pathname, _, err = c.cache.Cure(a); err != nil {
 		return
 	}
 
@@ -1526,7 +1531,11 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 		return
 	}
 
-	t := TContext{c, c.base.Append(dirWork, ids), c.base.Append(dirTemp, ids)}
+	t := TContext{
+		c.base.Append(dirWork, ids),
+		c.base.Append(dirTemp, ids),
+		common{c},
+	}
 	switch ca := a.(type) {
 	case TrivialArtifact:
 		defer t.destroy(&err)