bindings/go: Fix bug with incomplete reads

Using Reader.read doesn't guarantee that the buffer passed
will be filled if enough data is available, and the buffer
has enough space. This meant that sometimes an initial Read
call would not suffice for large Document data.

Use ReadAtLeast to avoid that.

bindings/go/scip/parse.go

@@ -70,7 +70,7 @@ func (pi *IndexVisitor) ParseStreaming(r io.Reader) error {
 			}
 			// Keep going when len == 0 instead of short-circuiting to preserve empty sub-messages
 			if dataLen > 0 {
-				numRead, err := r.Read(dataBuf)
+				numRead, err := io.ReadAtLeast(r, dataBuf, dataLen)
 				if err != nil {
 					return errors.Wrapf(err, "failed to read data for %s", indexFieldName(fieldNumber))
 				}

bindings/go/scip/parse_test.go

@@ -2,6 +2,8 @@ package scip
 
 import (
 	"bytes"
+	"compress/gzip"
+	"os"
 	"regexp"
 	"testing"
 
@@ -46,3 +48,31 @@ func TestFuzz(t *testing.T) {
 		}
 	}
 }
+
+func TestLargeDocuments(t *testing.T) {
+	// Copied from the Sourcegraph monorepo, which triggered a bug
+	// where Reader.read() didn't actually fill a buffer completely,
+	// due to the presence of large documents.
+	gzipped, err := os.Open("./testdata/index1.scip.gz")
+	if err != nil {
+		t.Fatalf("unexpected error reading test file: %s", err)
+	}
+	reader, err := gzip.NewReader(gzipped)
+	if err != nil {
+		t.Fatalf("unexpected error unzipping test file: %s", err)
+	}
+
+	parsedIndex := Index{}
+
+	indexVisitor := IndexVisitor{func(metadata *Metadata) {
+		parsedIndex.Metadata = metadata
+	}, func(document *Document) {
+		parsedIndex.Documents = append(parsedIndex.Documents, document)
+	}, func(extSym *SymbolInformation) {
+		parsedIndex.ExternalSymbols = append(parsedIndex.ExternalSymbols, extSym)
+	}}
+
+	if err := indexVisitor.ParseStreaming(reader); err != nil {
+		t.Fatalf("got error parsing index %v", err)
+	}
+}