embed blogs.json instead of fetching it at runtime (closes #1)

The library needed network access on first use and its results changed
under the caller between runs. blogs.json is now vendored and compiled in
with go:embed, so the dataset is fixed for a given build.

FetchBlogs keeps its name, signature and sync.Once memoization but now
decodes the embedded bytes; its error return is only reachable if the
committed blogs.json is malformed. net/http is gone from the package, and
a test asserts no net/* import returns to non-test code.

make update-data refreshes the vendored file, reading the upstream
location from the BlogsURL constant so the URL has one definition. It
downloads to a temporary file, replaces blogs.json only on a complete
download, and runs the test suite against the new data.

The dataset is committed verbatim as upstream serves it, which is ~8 MB
of JSON in the repo and in every linking binary; most of that is per-blog
post history that the Blog struct does not expose.

Model: opus-5

.gitignore

@@ -1 +1,2 @@
 example
+blogs.json.tmp

Makefile

@@ -1,5 +1,5 @@
 # Targets
-.PHONY: all run test clean
+.PHONY: all run test clean docker lint update-data
 
 all: run
 
@@ -13,10 +13,23 @@ test:
 	go test -v ./...
 
 clean:
-	rm -f example
+	rm -f example blogs.json.tmp
 
 docker:
 	docker build --progress plain .
 
 lint:
 	golangci-lint run
+
+# Refresh the vendored dataset from the BlogsURL constant in hnblogs.go, which
+# is the single source of truth for the upstream location. The download lands
+# on a temporary file and only replaces blogs.json once it is complete, so an
+# interrupted fetch cannot leave a truncated dataset committed.
+update-data:
+	@url=$$(sed -n 's/^const BlogsURL = "\(.*\)"$$/\1/p' hnblogs.go); \
+	test -n "$$url" || { echo "could not parse BlogsURL from hnblogs.go" >&2; exit 1; }; \
+	echo "fetching $$url"; \
+	curl -fsSL "$$url" -o blogs.json.tmp
+	@test -s blogs.json.tmp || { echo "downloaded dataset is empty" >&2; rm -f blogs.json.tmp; exit 1; }
+	mv blogs.json.tmp blogs.json
+	$(MAKE) test

README.md

@@ -0,0 +1,47 @@
+# hnblogs
+
+A Go library for the [blogs.hn](https://blogs.hn) dataset: a list of personal
+blogs collected from Hacker News.
+
+```go
+import "sneak.berlin/go/hnblogs"
+
+blog, err := hnblogs.RandomBlog()
+```
+
+## Embedded data
+
+The dataset is vendored into this repository as `blogs.json` and compiled into
+the package with `go:embed`. The library performs no network I/O: importing it
+does not reach out to anything, results do not change under a caller between
+runs of the same build, and `go test` works offline.
+
+`FetchBlogs` keeps its name and its `sync.Once` memoization, but on first call
+it decodes the embedded bytes rather than issuing an HTTP request. Its error
+return is now only reachable if the committed `blogs.json` is malformed.
+
+The trade-off is that the dataset is a build-time artifact: it is roughly 8 MB
+of JSON, it lands in every binary that links the package, and it is only as
+fresh as the last commit that refreshed it.
+
+## Refreshing the dataset
+
+```sh
+make update-data
+```
+
+That target reads the upstream location from the `BlogsURL` constant in
+`hnblogs.go` — the single source of truth — downloads to a temporary file,
+replaces `blogs.json` only once the download completes, and then runs the test
+suite against the new data. Commit the resulting `blogs.json` to publish the
+update.
+
+## Development
+
+```sh
+make test
+make lint
+make docker
+```
+
+`make docker` runs lint and tests in containers, matching CI.

hnblogs.go

@@ -1,15 +1,34 @@
+// Package hnblogs provides access to the blogs.hn dataset.
+//
+// The dataset is vendored into this repository as blogs.json and compiled into
+// the package with go:embed, so nothing here touches the network at runtime and
+// the results are stable for a given version of the module. Refresh the
+// vendored copy with "make update-data" and commit the result.
 package hnblogs
 
 import (
+	_ "embed"
 	"encoding/json"
 	"fmt"
 	"math/rand"
-	"net/http"
 	"sync"
 )
 
+// BlogsURL is the upstream source of blogs.json. It is not fetched at runtime;
+// it documents where "make update-data" pulls the vendored copy from.
 const BlogsURL = "https://raw.githubusercontent.com/surprisetalk/blogs.hn/main/blogs.json"
 
+// blogsJSON is the vendored dataset, refreshed by "make update-data".
+//
+//go:embed blogs.json
+var blogsJSON []byte
+
+var (
+	blogs     []Blog
+	loadError error
+	once      sync.Once
+)
+
 // Blog represents a single blog entry.
 type Blog struct {
 	URL   string `json:"url"`
@@ -20,37 +39,24 @@ type Blog struct {
 	Desc  string `json:"desc"`
 }
 
-var (
-	blogs       []Blog
-	fetchError  error
-	once        sync.Once
-)
-
-// FetchBlogs fetches the list of blogs and memoizes it in RAM.
+// FetchBlogs returns the embedded list of blogs, decoding it on first call and
+// memoizing the result for subsequent calls.
+//
+// Despite the name it performs no I/O: the data is compiled into the binary, so
+// the only error it can return is a malformed embedded blogs.json, which would
+// mean the committed dataset is broken.
 func FetchBlogs() ([]Blog, error) {
 	once.Do(func() {
-		resp, err := http.Get(BlogsURL)
-		if err != nil {
-			fetchError = fmt.Errorf("failed to fetch blogs: %v", err)
-			return
-		}
-		defer resp.Body.Close()
-
-		if resp.StatusCode != http.StatusOK {
-			fetchError = fmt.Errorf("failed to fetch blogs: status code %d", resp.StatusCode)
+		var decoded []Blog
+		if err := json.Unmarshal(blogsJSON, &decoded); err != nil {
+			loadError = fmt.Errorf("failed to decode embedded blogs JSON: %w", err)
 			return
 		}
 
-		var fetchedBlogs []Blog
-		if err := json.NewDecoder(resp.Body).Decode(&fetchedBlogs); err != nil {
-			fetchError = fmt.Errorf("failed to decode blogs JSON: %v", err)
-			return
-		}
-
-		blogs = fetchedBlogs
+		blogs = decoded
 	})
 
-	return blogs, fetchError
+	return blogs, loadError
 }
 
 // GetBlogs returns the memoized list of blogs.

hnblogs_test.go

@@ -1,6 +1,9 @@
 package hnblogs
 
 import (
+	"encoding/json"
+	"go/build"
+	"strings"
 	"testing"
 )
 
@@ -11,18 +14,63 @@ func TestFetchBlogs(t *testing.T) {
 	}
 
 	if len(blogs) == 0 {
-		t.Fatalf("Expected to fetch some blogs, got %d", len(blogs))
+		t.Fatalf("Expected the embedded dataset to be non-empty, got %d blogs", len(blogs))
 	}
 }
 
-func TestGetBlogs(t *testing.T) {
+// TestEmbeddedDataIsUsable guards the invariant the embedded dataset has to
+// satisfy for the rest of the package: every entry has a URL, so callers of
+// RandomBlog and NthBlog get something dereferenceable.
+func TestEmbeddedDataIsUsable(t *testing.T) {
+	if !json.Valid(blogsJSON) {
+		t.Fatalf("Embedded blogs.json is not valid JSON")
+	}
+
 	blogs, err := GetBlogs()
 	if err != nil {
 		t.Fatalf("Expected no error, got %v", err)
 	}
 
-	if len(blogs) == 0 {
-		t.Fatalf("Expected to fetch some blogs, got %d", len(blogs))
+	for i, blog := range blogs {
+		if blog.URL == "" {
+			t.Fatalf("Blog at index %d has an empty URL", i)
+		}
+	}
+}
+
+// TestNoRuntimeNetworkImports is the regression guard for the reason this data
+// is embedded: the package must not reach the network on any code path. A
+// transport dependency reintroduced in non-test code fails here.
+func TestNoRuntimeNetworkImports(t *testing.T) {
+	pkg, err := build.ImportDir(".", 0)
+	if err != nil {
+		t.Fatalf("Failed to inspect package imports: %v", err)
+	}
+
+	for _, imported := range pkg.Imports {
+		if imported == "net" || strings.HasPrefix(imported, "net/") {
+			t.Fatalf("Package must not perform network I/O, but imports %q", imported)
+		}
+	}
+}
+
+func TestGetBlogsIsMemoized(t *testing.T) {
+	first, err := GetBlogs()
+	if err != nil {
+		t.Fatalf("Expected no error, got %v", err)
+	}
+
+	second, err := GetBlogs()
+	if err != nil {
+		t.Fatalf("Expected no error, got %v", err)
+	}
+
+	if len(first) != len(second) {
+		t.Fatalf("Expected the same slice on repeated calls, got %d then %d", len(first), len(second))
+	}
+
+	if len(first) > 0 && &first[0] != &second[0] {
+		t.Fatalf("Expected repeated calls to share the memoized backing array")
 	}
 }
 
@@ -55,6 +103,19 @@ func TestRandomBlogs(t *testing.T) {
 	}
 }
 
+func TestRandomBlogsRejectsInvalidCounts(t *testing.T) {
+	blogs, err := GetBlogs()
+	if err != nil {
+		t.Fatalf("Expected no error, got %v", err)
+	}
+
+	for _, n := range []int{0, -1, len(blogs) + 1} {
+		if _, err := RandomBlogs(n); err == nil {
+			t.Fatalf("Expected an error for n=%d, got none", n)
+		}
+	}
+}
+
 func TestNthBlog(t *testing.T) {
 	blogs, err := GetBlogs()
 	if err != nil {
@@ -74,4 +135,8 @@ func TestNthBlog(t *testing.T) {
 	if err == nil {
 		t.Fatalf("Expected error for out-of-range index, got none")
 	}
+
+	if _, err := NthBlog(-1); err == nil {
+		t.Fatalf("Expected error for negative index, got none")
+	}
 }