watchpr: emit the baseline under --json

The baseline is the only notice that a watch started against an already
conflicted PR, and it was missing from the mode automation uses. It goes to
stderr as a JSON record, leaving stdout a single result record.

cmd/watchpr/main.go

@@ -3,8 +3,13 @@
 // new comment from someone other than the agent, its CI fails, or it loses
 // mergeability after the baseline. Benign transitions (CI pending→success, the
 // agent's own pushes and comments) are ignored, and so is any condition that was
-// already true at the baseline -- which watchpr prints, so a run started against
-// a conflicted or CI-red PR says so.
+// already true at the baseline -- which watchpr prints on stderr (as a JSON
+// record under --json), so a run started against a conflicted or CI-red PR says
+// so. One case is never reported at all: a conflict introduced by the push
+// immediately before the watch started, on a PR whose head and base then never
+// move, stays silent for the whole run, because nothing in Gitea's payload
+// separates it from a merge check still in flight. The baseline line is the
+// only notice of it.
 //
 //	watchpr owner/repo#12 owner/repo:15
 //	watchpr --once --json owner/repo#12
@@ -15,6 +20,7 @@ package main
 import (
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
 	"strings"
 	"time"
@@ -131,13 +137,7 @@ func runWatch(c *agent.GiteaClient, refs []agent.PRRef, interval time.Duration,
 	defer ticker.Stop()
 
 	onBaseline := func(states []agent.PRState) {
-		if jsonMode {
-			return
-		}
-		fmt.Fprintf(os.Stderr, "watching %d PR(s) every %s; baseline established\n", len(refs), interval)
-		for _, st := range states {
-			fmt.Fprintln(os.Stderr, baselineLine(st))
-		}
+		emitBaselines(os.Stderr, states, interval, jsonMode)
 	}
 	onError := func(ref agent.PRRef, err error) {
 		fmt.Fprintf(os.Stderr, "warning: polling %s: %v\n", ref.String(), err)
@@ -151,6 +151,32 @@ func runWatch(c *agent.GiteaClient, refs []agent.PRRef, interval time.Duration,
 	return nil
 }
 
+// baselineRecord is the --json form of baselineLine. Both go to stderr, leaving
+// the stdout contract a single result record: a caller automating watchpr is
+// precisely the one who needs to be told the watch started against a PR that is
+// already conflicted, since no alert will ever follow for it.
+type baselineRecord struct {
+	Baseline   bool          `json:"baseline"`
+	Suppressed string        `json:"suppressed,omitempty"`
+	State      agent.PRState `json:"state"`
+}
+
+// emitBaselines writes the state each watch started from, in whichever form the
+// caller asked for.
+func emitBaselines(w io.Writer, states []agent.PRState, interval time.Duration, jsonMode bool) {
+	if jsonMode {
+		enc := json.NewEncoder(w)
+		for _, st := range states {
+			_ = enc.Encode(baselineRecord{Baseline: true, Suppressed: suppressedAtBaseline(st), State: st})
+		}
+		return
+	}
+	_, _ = fmt.Fprintf(w, "watching %d PR(s) every %s; baseline established\n", len(states), interval)
+	for _, st := range states {
+		_, _ = fmt.Fprintln(w, baselineLine(st))
+	}
+}
+
 // describeFailure names the cause of a terminal failure so a watcher that stops
 // says why. An anonymous rejection, a permission boundary and a token that
 // outlived its Vault lease are three different problems and only the last is

cmd/watchpr/main_test.go

@@ -1,6 +1,8 @@
 package main
 
 import (
+	"bytes"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -8,6 +10,7 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 
 	"git.unkin.net/unkin/agent-tools/internal/agent"
 )
@@ -212,3 +215,44 @@ func TestBaselineLineNamesSuppressedConditions(t *testing.T) {
 		t.Errorf("baselineLine = %q, want no suppression note for a clean baseline", got)
 	}
 }
+
+// --json is the mode automation uses, and automation is exactly who needs to be
+// told the watch started against an already-conflicted PR -- the one case that
+// will never produce an alert. It must therefore be emitted in JSON mode too,
+// on stderr, where it cannot corrupt the result record on stdout.
+func TestBaselineIsEmittedInJSONMode(t *testing.T) {
+	st := agent.PRState{
+		Ref:       agent.PRRef{Owner: "unkin", Repo: "repo", Number: 7},
+		State:     "open",
+		Mergeable: agent.MergeNo,
+		CIStatus:  "failure",
+		HeadSHA:   "cafebabecafebabe",
+	}
+	var buf bytes.Buffer
+	emitBaselines(&buf, []agent.PRState{st}, 30*time.Second, true)
+
+	var got baselineRecord
+	if err := json.Unmarshal(buf.Bytes(), &got); err != nil {
+		t.Fatalf("baseline is not a JSON record (%v); got %q", err, buf.String())
+	}
+	if !got.Baseline {
+		t.Error("record does not mark itself as the baseline")
+	}
+	if got.State.Ref != st.Ref || got.State.Mergeable != agent.MergeNo {
+		t.Errorf("record state = %+v, want the conflicted snapshot the watch started from", got.State)
+	}
+	for _, want := range []string{"already non-mergeable", "CI already failure"} {
+		if !strings.Contains(got.Suppressed, want) {
+			t.Errorf("suppressed = %q, want it to mention %q", got.Suppressed, want)
+		}
+	}
+
+	buf.Reset()
+	clean := st
+	clean.Mergeable = agent.MergeYes
+	clean.CIStatus = "success"
+	emitBaselines(&buf, []agent.PRState{clean}, 30*time.Second, true)
+	if strings.Contains(buf.String(), "suppressed") {
+		t.Errorf("clean baseline = %q, want no suppression field", buf.String())
+	}
+}