derivation: parse nix derivation show

This time it's JSON, and actual intended behaviour.

build.go

@@ -2,9 +2,9 @@ package nixbuild
 
 import (
 	"context"
+	"errors"
 	"io"
 	"iter"
-	"strings"
 )
 
 // Build builds all entries yielded by installables.
@@ -32,18 +32,11 @@ func Build(ctx context.Context, installables iter.Seq[string]) error {
 		return err
 	}
 
-	for drv := range installables {
-		if strings.HasSuffix(drv, ".drv") {
-			// this is just what nix requires now :c
-			drv += "^*"
-		}
-		if _, err := iw.Write([]byte(drv + "\n")); err != nil {
-			return err
-		}
+	if _, err := WriteStdin(iw, installables); err != nil {
+		return errors.Join(err, cmd.Wait())
 	}
-
 	if err := iw.Close(); err != nil {
-		return err
+		return errors.Join(err, cmd.Wait())
 	}
 
 	return cmd.Wait()

cmd/nixbuild/main.go

@@ -42,7 +42,20 @@ func main() {
 		Flag(&flagVerbose, "v", command.BoolFlag(false), "Connect nix stderr").
 		Flag(&flagJSON, "json", command.BoolFlag(false), "Serialise output in JSON when applicable")
 
-	c.Command("build", "Build a list of installables", func(args []string) error {
+	c.Command("show", command.UsageInternal, func(args []string) error {
+		if len(args) < 1 {
+			return commandHandlerError("usage requires at least 1 argument")
+		}
+
+		if drv, err := nixbuild.DerivationShow(ctx, slices.Values(args)); err != nil {
+			return commandHandlerError(fmt.Sprintf("cannot show: %v", err))
+		} else {
+			log.Printf("got %d derivations:\n%#v", len(drv), drv)
+			return nil
+		}
+	})
+
+	c.Command("build", command.UsageInternal, func(args []string) error {
 		if len(args) < 1 {
 			return commandHandlerError("build requires at least 1 argument")
 		}

derivation.go

@@ -0,0 +1,88 @@
+package nixbuild
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"iter"
+)
+
+type (
+	// DerivationMap is the output of `nix derivation show`.
+	DerivationMap map[string]Derivation
+
+	// Derivation is a description of a derivation.
+	Derivation struct {
+		// Args are arguments passed to Builder.
+		Args []string `json:"args"`
+		// Builder is the store path of the program that builds the [Derivation].
+		Builder string `json:"builder"`
+		// System is the value of pkgs.system during evaluation.
+		System string `json:"system"`
+		// Name is the user-facing name of a derivation, as seen in the nix store path suffix.
+		Name string `json:"name"`
+
+		Environment      json.RawMessage     `json:"env"`
+		InputDerivations InputDerivationsMap `json:"inputDrvs"`
+		InputSources     []string            `json:"inputSrcs"`
+		Outputs          OutputsMap          `json:"outputs"`
+	}
+
+	// OutputsMap is an output name to [Output] map.
+	OutputsMap map[string]Output
+
+	// Output is an output of a [Derivation].
+	Output struct {
+		Path string `json:"path"`
+	}
+
+	// InputDerivationsMap is a store path to metadata map.
+	InputDerivationsMap map[string]InputDerivation
+
+	// InputDerivation contains input derivation metadata.
+	InputDerivation struct {
+		DynamicOutputs json.RawMessage `json:"dynamicOutputs"`
+		Outputs        []string        `json:"outputs"`
+	}
+)
+
+// DerivationShow returns a [DerivationMap] describing all entries yielded by installables.
+func DerivationShow(ctx context.Context, installables iter.Seq[string]) (DerivationMap, error) {
+	c, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	cmd := Nix(c, CommandDerivation, CommandDerivationShow,
+		FlagStdin)
+	ir, iw := io.Pipe()
+	cmd.Stdin = ir
+	or, ow := io.Pipe()
+	cmd.Stdout = ow
+	if Stderr != nil {
+		cmd.Stderr = Stderr
+	}
+	cmd.Cancel = func() error { _ = ow.Close(); return cmd.Process.Kill() }
+
+	if err := cmd.Start(); err != nil {
+		return nil, err
+	}
+
+	done := make(chan error)
+	var v DerivationMap
+	go func() { done <- json.NewDecoder(or).Decode(&v) }()
+
+	if _, err := WriteStdin(iw, installables); err != nil {
+		return nil, errors.Join(err, cmd.Wait())
+	}
+	if err := iw.Close(); err != nil {
+		return nil, errors.Join(err, cmd.Wait())
+	}
+
+	if err := cmd.Wait(); err != nil {
+		// deferred cancel closes pipe
+		return nil, err
+	}
+
+	err := <-done
+	return v, err
+}

format.go

@@ -2,12 +2,21 @@ package nixbuild
 
 import (
 	"context"
+	"io"
+	"iter"
 	"os/exec"
+	"strings"
 )
 
 const (
 	// CommandBuild build a derivation or fetch a store path
 	CommandBuild = "build"
+	// CommandDerivation work with derivations, Nix's notion of a build plan.
+	CommandDerivation = "derivation"
+	// CommandDerivationAdd add a store derivation
+	CommandDerivationAdd = "add"
+	// CommandDerivationShow show the contents of a store derivation
+	CommandDerivationShow = "show"
 
 	// FlagDryRun show what this command would do without doing it.
 	FlagDryRun = "--dry-run"
@@ -86,3 +95,20 @@ const (
 
 // NixOSInstallable returns the nixos installable for a given flake and host.
 func NixOSInstallable(flake, host string) string { return flake + nixosSuffix0 + host + nixosSuffix1 }
+
+// WriteStdin writes installables for a nix process running with [FlagStdin].
+func WriteStdin(w io.Writer, installables iter.Seq[string]) (int, error) {
+	var count int
+	for drv := range installables {
+		if strings.HasSuffix(drv, ".drv") {
+			// this is just what nix requires now :c
+			drv += "^*"
+		}
+		n, err := w.Write([]byte(drv + "\n"))
+		count += n
+		if err != nil {
+			return count, err
+		}
+	}
+	return count, nil
+}