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.go

@@ -149,27 +149,63 @@ func tryDownloadPayload(payload Payload, dest, fileID string, allowHashMismatch
 
 var downloadHTTPClient = &http.Client{Timeout: 30 * time.Minute}
 
+// httpDownloadFile downloads url to dest via a dest+".part" temp file,
+// resuming from wherever a previous attempt left off if one exists - MSVC/
+// WinSDK/WDK/DXSDK payloads run into the hundreds of MB to multiple GB, so
+// restarting an interrupted download from byte 0 (a dropped connection, a
+// retry after this same function returned an error) wastes real time and
+// bandwidth on a flaky connection. Requests a byte Range starting at the
+// existing .part file's size, if any; a server that doesn't honor Range
+// (responds 200 instead of 206) gets treated as sending the whole file
+// again from byte 0, so the .part is truncated and started over rather
+// than getting byte-0 content appended onto existing bytes.
 func httpDownloadFile(url, dest string) error {
-	resp, err := downloadHTTPClient.Get(url)
+	tmp := dest + ".part"
+	var offset int64
+	if fi, err := os.Stat(tmp); err == nil {
+		offset = fi.Size()
+	}
+
+	req, err := http.NewRequest(http.MethodGet, url, nil)
+	if err != nil {
+		return err
+	}
+	if offset > 0 {
+		req.Header.Set("Range", fmt.Sprintf("bytes=%d-", offset))
+	}
+	resp, err := downloadHTTPClient.Do(req)
 	if err != nil {
 		return err
 	}
 	defer resp.Body.Close()
-	if resp.StatusCode != http.StatusOK {
+
+	var out *os.File
+	switch resp.StatusCode {
+	case http.StatusOK:
+		// No partial-content support (or nothing to resume from): the body
+		// is the whole file from byte 0.
+		out, err = os.Create(tmp)
+	case http.StatusPartialContent:
+		out, err = os.OpenFile(tmp, os.O_WRONLY|os.O_APPEND, 0o644)
+	case http.StatusRequestedRangeNotSatisfiable:
+		// Our .part is already >= the real file size - stale or corrupt.
+		// Discard it; the next retry starts clean with no Range header.
+		os.Remove(tmp)
+		return fmt.Errorf("GET %s: range not satisfiable, discarding partial download and retrying from scratch", url)
+	default:
 		return fmt.Errorf("GET %s: %s", url, resp.Status)
 	}
-	tmp := dest + ".part"
-	out, err := os.Create(tmp)
 	if err != nil {
 		return err
 	}
+
 	if _, err := io.Copy(out, resp.Body); err != nil {
 		out.Close()
-		os.Remove(tmp)
+		// Deliberately not removing tmp here: whatever bytes made it to
+		// disk are exactly what the next attempt should resume from.
 		return err
 	}
 	if err := out.Close(); err != nil {
-		os.Remove(tmp)
 		return err
 	}
 	return os.Rename(tmp, dest)

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)
+	}
+}