cmd/cue: do not panic on empty jsonl inputs

buildPlan.instances always returned an iterator with at least
one instance, and in the case of an empty jsonl file,
this would result in iterating over a single nil instance.
This would cause the following logic, like exporting, to panic.

An empty jsonl input means zero files, and is allowed by the spec.
Moreover, zero bytes is not valid input, unlike YAML or TOML,
which produce null and {} respectively.
For these reasons, treat empty jsonl inputs as a stream of zero values.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ie0a62bb59d347011de5aa47f5ee6d175824451a3
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199451
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

cmd/cue/cmd/common.go

@@ -142,12 +142,18 @@ func (b *buildPlan) instances() iterator {
 			e:    err,
 			i:    -1,
 		}
-	default:
+	case b.instance != nil:
 		i = &instanceIterator{
 			a: []*instance{b.instance},
 			i: -1,
 		}
 		b.instance = nil
+	default:
+		// No instances; return an iterator with zero values.
+		// This can happen when the input is an empty jsonl file, for example.
+		// Zero iteration may not make much sense in some scenarios like export,
+		// but an empty jsonl file is valid, so doing nothing seems reasonable.
+		i = &instanceIterator{}
 	}
 	if len(b.expressions) > 0 {
 		return &expressionIter{

cmd/cue/cmd/testdata/script/encoding_empty.txtar

@@ -1,6 +1,7 @@
 # Test that the various encodings cope with empty files correctly.
-
-# TODO(mvdan): fix the panics below; see https://cuelang.org/issue/1790 and https://cuelang.org/issue/2714.
+# Note that empty files or inputs have different meanings per encoding.
+# For example, in CUE and TOML they mean an empty struct or object,
+# in JSON they are invalid, and in JSONL they are a stream of zero values.
 
 # TODO(mvdan): cover more encodings: jsonschema, openapi, textproto, proto.
 
@@ -12,8 +13,8 @@ exec cue export cue: empty
 cmp stdout as-cue.stdout
 ! exec cue export json: empty
 stderr 'unexpected end of JSON input'
-! exec cue export jsonl: empty
-stderr '^panic: '
+exec cue export jsonl: empty
+cmp stdout as-jsonl.stdout
 exec cue export yaml: empty
 cmp stdout as-yaml.stdout
 exec cue export toml: empty
@@ -25,8 +26,8 @@ exec cue export cue: newlines
 cmp stdout as-cue.stdout
 ! exec cue export json: newlines
 stderr 'unexpected end of JSON input'
-! exec cue export jsonl: newlines
-stderr '^panic: '
+exec cue export jsonl: newlines
+cmp stdout as-jsonl.stdout
 exec cue export yaml: newlines
 cmp stdout as-yaml.stdout
 exec cue export toml: newlines
@@ -48,6 +49,7 @@ cmp stdout as-toml.stdout
 
 -- as-cue.stdout --
 {}
+-- as-jsonl.stdout --
 -- as-yaml.stdout --
 null
 -- as-toml.stdout --