narinfo: implement parser :D

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ic10f3e69f18308d620b1461fd62d46386a6a6964

internal/narinfo/narinfo.go

@@ -0,0 +1,81 @@
+package narinfo
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+)
+
+// Parsed representation of a Nix narinfo file.
+type NarInfo struct {
+	StorePath   string
+	URL         string
+	Compression string
+	FileHash    string
+	FileSize    uint64
+	NarHash     string
+	NarSize     uint64
+	References  []string
+	Deriver     string
+	Sig         []string
+	CA          string
+}
+
+// Parses a narinfo from r. Returns error on malformed input or missing StorePath.
+func Parse(r io.Reader) (*NarInfo, error) {
+	ni := &NarInfo{}
+	scanner := bufio.NewScanner(r)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			continue
+		}
+		k, v, ok := strings.Cut(line, ": ")
+		if !ok {
+			return nil, fmt.Errorf("malformed line: %q", line)
+		}
+		switch k {
+		case "StorePath":
+			ni.StorePath = v
+		case "URL":
+			ni.URL = v
+		case "Compression":
+			ni.Compression = v
+		case "FileHash":
+			ni.FileHash = v
+		case "FileSize":
+			n, err := strconv.ParseUint(v, 10, 64)
+			if err != nil {
+				return nil, fmt.Errorf("FileSize: %w", err)
+			}
+			ni.FileSize = n
+		case "NarHash":
+			ni.NarHash = v
+		case "NarSize":
+			n, err := strconv.ParseUint(v, 10, 64)
+			if err != nil {
+				return nil, fmt.Errorf("NarSize: %w", err)
+			}
+			ni.NarSize = n
+		case "References":
+			if v != "" {
+				ni.References = strings.Fields(v)
+			}
+		case "Deriver":
+			ni.Deriver = v
+		case "Sig":
+			ni.Sig = append(ni.Sig, v)
+		case "CA":
+			ni.CA = v
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+	if ni.StorePath == "" {
+		return nil, fmt.Errorf("missing StorePath")
+	}
+	return ni, nil
+}