Resume interrupted downloads instead of restarting from scratch

MSVC/WinSDK/WDK/DXSDK payloads run into the hundreds of MB to several
GB, so a dropped connection or a retry after a transient error used to
mean throwing away everything already fetched and starting over from
byte 0. Track progress in a dest+".part" file and resume it via an
HTTP Range request, falling back to a full restart when the server
doesn't honor Range (200 instead of 206) or the local part is stale
(416).

internal/download/fetch_resume_test.go

@@ -0,0 +1,173 @@
+package download
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestHTTPDownloadFileFullDownload(t *testing.T) {
+	const body = "the quick brown fox jumps over the lazy dog"
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, body)
+	}))
+	defer srv.Close()
+
+	dest := filepath.Join(t.TempDir(), "out")
+	if err := httpDownloadFile(srv.URL, dest); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(dest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != body {
+		t.Errorf("downloaded content = %q, want %q", got, body)
+	}
+}
+
+// rangeServer serves a fixed body and honors byte-range requests, exactly
+// like a real payload host (GitHub Releases, nuget.org, etc.) would.
+func rangeServer(body string) *httptest.Server {
+	return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		rng := r.Header.Get("Range")
+		if rng == "" {
+			w.WriteHeader(http.StatusOK)
+			fmt.Fprint(w, body)
+			return
+		}
+		var start int
+		if _, err := fmt.Sscanf(rng, "bytes=%d-", &start); err != nil || start < 0 || start > len(body) {
+			w.WriteHeader(http.StatusRequestedRangeNotSatisfiable)
+			return
+		}
+		w.Header().Set("Content-Range", "bytes "+strconv.Itoa(start)+"-"+strconv.Itoa(len(body)-1)+"/"+strconv.Itoa(len(body)))
+		w.WriteHeader(http.StatusPartialContent)
+		fmt.Fprint(w, body[start:])
+	}))
+}
+
+func TestHTTPDownloadFileResumesFromExistingPart(t *testing.T) {
+	const body = "the quick brown fox jumps over the lazy dog"
+	srv := rangeServer(body)
+	defer srv.Close()
+
+	dest := filepath.Join(t.TempDir(), "out")
+	partial := body[:10]
+	if err := os.WriteFile(dest+".part", []byte(partial), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := httpDownloadFile(srv.URL, dest); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(dest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != body {
+		t.Errorf("resumed download content = %q, want %q (partial %q should have been continued, not duplicated or lost)", got, body, partial)
+	}
+}
+
+func TestHTTPDownloadFileRestartsWhenServerIgnoresRange(t *testing.T) {
+	const body = "the quick brown fox jumps over the lazy dog"
+	// Always answers 200 with the full body, regardless of Range - some
+	// servers/CDNs genuinely don't support partial content.
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, body)
+	}))
+	defer srv.Close()
+
+	dest := filepath.Join(t.TempDir(), "out")
+	// A stale/bogus .part that must NOT end up prepended to the real
+	// content - if httpDownloadFile appended instead of truncating here,
+	// the result would start with this garbage.
+	if err := os.WriteFile(dest+".part", []byte("GARBAGE-FROM-A-STALE-ATTEMPT"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := httpDownloadFile(srv.URL, dest); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(dest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != body {
+		t.Errorf("content = %q, want exactly %q (no leftover garbage prepended)", got, body)
+	}
+}
+
+func TestHTTPDownloadFileKeepsPartOnMidTransferFailure(t *testing.T) {
+	const fullBody = "0123456789"
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, fullBody[:5])
+		if f, ok := w.(http.Flusher); ok {
+			f.Flush()
+		}
+		// Simulate a dropped connection partway through by closing the
+		// underlying connection abruptly instead of finishing the body.
+		hj, ok := w.(http.Hijacker)
+		if !ok {
+			return
+		}
+		conn, _, err := hj.Hijack()
+		if err == nil {
+			conn.Close()
+		}
+	}))
+	defer srv.Close()
+
+	dest := filepath.Join(t.TempDir(), "out")
+	err := httpDownloadFile(srv.URL, dest)
+	if err == nil {
+		t.Fatal("expected an error from the truncated connection")
+	}
+
+	partial, err := os.ReadFile(dest + ".part")
+	if err != nil {
+		t.Fatalf("expected the .part file with the bytes received so far to survive a failed download: %v", err)
+	}
+	if !strings.HasPrefix(fullBody, string(partial)) || len(partial) == 0 {
+		t.Errorf(".part content = %q, want a non-empty prefix of %q", partial, fullBody)
+	}
+}
+
+func TestHTTPDownloadFileRangeNotSatisfiableDiscardsPart(t *testing.T) {
+	const body = "short"
+	srv := rangeServer(body)
+	defer srv.Close()
+
+	dest := filepath.Join(t.TempDir(), "out")
+	// .part is already longer than the real file - triggers 416 from
+	// rangeServer's own bounds check.
+	if err := os.WriteFile(dest+".part", []byte("this partial file is way too long"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := httpDownloadFile(srv.URL, dest); err == nil {
+		t.Fatal("expected an error on the first (416) attempt")
+	}
+	if _, err := os.Stat(dest + ".part"); !os.IsNotExist(err) {
+		t.Error("expected the stale .part to be discarded after a 416 response")
+	}
+
+	// The retry (a fresh caller, no Range header since .part is gone) should
+	// now succeed cleanly.
+	if err := httpDownloadFile(srv.URL, dest); err != nil {
+		t.Fatalf("retry after discarding the stale .part failed: %v", err)
+	}
+	got, err := os.ReadFile(dest)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != body {
+		t.Errorf("content = %q, want %q", got, body)
+	}
+}