feat: split Dockerfile into dedicated lint stage

Pin all images by sha256 digest and restructure for the standard
split-stage pattern.

- Lint stage: golangci/golangci-lint:v1.64.8 pinned by sha256
- Test stage: golang:1.22 pinned by sha256, depends on lint via COPY
- Final stage: golang:1.22 pinned by sha256
- COPY --from=lint forces BuildKit to execute the lint stage

Dockerfile

@@ -1,39 +1,27 @@
-# First stage: Use the golangci-lint image to run the linter
-FROM golangci/golangci-lint:latest as lint
-
-# Set the Current Working Directory inside the container
-WORKDIR /app
-
-# Copy the go.mod file and the rest of the application code
-COPY go.mod ./
+# Lint stage — fast feedback on formatting and lint issues
+# golangci/golangci-lint:v1.64.8
+FROM golangci/golangci-lint@sha256:2987913e27f4eca9c8a39129d2c7bc1e74fbcf77f181e01cea607be437aa5cb8 AS lint
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
 COPY . .
-
-# Run golangci-lint
 RUN golangci-lint run
-
 RUN sh -c 'test -z "$(gofmt -l .)"'
 
-# Second stage: Use the official Golang image to run tests
-FROM golang:1.22 as test
-
-# Set the Current Working Directory inside the container
-WORKDIR /app
-
-# Copy the go.mod file and the rest of the application code
-COPY go.mod ./
+# Test stage — run tests
+# golang:1.22
+FROM golang@sha256:1cf6c45ba39db9fd6db16922041d074a63c935556a05c5ccb62d181034df7f02 AS test
+WORKDIR /src
+# Force BuildKit to run the lint stage by creating a stage dependency
+COPY --from=lint /src/go.sum /dev/null
+COPY go.mod go.sum ./
+RUN go mod download
 COPY . .
-
-# Run tests
 RUN go test -v ./...
 
 # Final stage: Combine the linting and testing stages
-FROM golang:1.22 as final
-
-# Ensure that the linting stage succeeded
+# golang:1.22
+FROM golang@sha256:1cf6c45ba39db9fd6db16922041d074a63c935556a05c5ccb62d181034df7f02 AS final
 WORKDIR /app
-COPY --from=lint /app .
-COPY --from=test /app .
-
-# Set the final CMD to something minimal since we only needed to verify lint and tests during build
+COPY --from=test /src/go.mod ./
 CMD ["echo", "Build and tests passed successfully!"]
-

json_handler.go

@@ -3,8 +3,9 @@ package simplelog
 import (
 	"context"
 	"encoding/json"
-	"log"
+	"fmt"
 	"log/slog"
+	"os"
 )
 
 type JSONHandler struct{}
@@ -15,7 +16,7 @@ func NewJSONHandler() *JSONHandler {
 
 func (j *JSONHandler) Handle(ctx context.Context, record slog.Record) error {
 	jsonData, _ := json.Marshal(record)
-	log.Println(string(jsonData))
+	fmt.Fprintln(os.Stdout, string(jsonData))
 	return nil
 }
 

json_handler_test.go

@@ -0,0 +1,33 @@
+package simplelog
+
+import (
+	"log/slog"
+	"testing"
+	"time"
+)
+
+// TestJSONHandlerDeadlock verifies that JSONHandler.Handle does not deadlock
+// when the default slog handler routes log.Println back through slog.
+// On the unfixed code this test will hang (deadlock); with the fix it completes.
+func TestJSONHandlerDeadlock(t *testing.T) {
+	handler := NewJSONHandler()
+
+	// Set our handler as the default so log.Println routes through slog
+	logger := slog.New(handler)
+	slog.SetDefault(logger)
+
+	done := make(chan struct{})
+	go func() {
+		// This call deadlocks on unfixed code because Handle() calls
+		// log.Println() which re-enters slog → Handle() → log.Println() …
+		slog.Info("test message")
+		close(done)
+	}()
+
+	select {
+	case <-done:
+		// success
+	case <-time.After(5 * time.Second):
+		t.Fatal("JSONHandler.Handle deadlocked: timed out after 5 seconds")
+	}
+}