fix: detect silent wget failures in container DownloadFile

DownloadFile discarded wget's stdout/stderr, so failed downloads
(e.g. unreachable host) produced no error. The caller then tried to
restore a missing or empty .bak file, yielding a confusing 'volume
is empty' message from SQL Server.

After wget completes, verify the output file exists and is non-empty
via 'test -s'. If missing, panic with the wget stderr for diagnosis.

Also adds mkdir -p for idempotent directory creation.

Fixes #566

Author: dlevy-msft-sql

internal/container/controller.go

@@ -250,20 +250,25 @@ func (c Controller) DownloadFile(id string, src string, destFolder string) {
 		panic("Must pass in non-empty destFolder")
 	}
 
-	cmd := []string{"mkdir", destFolder}
+	cmd := []string{"mkdir", "-p", destFolder}
 	c.runCmdInContainer(id, cmd)
 
 	_, file := filepath.Split(src)
-
-	// Wget the .bak file from the http src, and place it in /var/opt/sql/backup
-	cmd = []string{
-		"wget",
-		"-O",
-		destFolder + "/" + file, // not using filepath.Join here, this is in the *nix container. always /
-		src,
+	dest := destFolder + "/" + file // not using filepath.Join here, this is in the *nix container. always /
+
+	cmd = []string{"wget", "-O", dest, src}
+	_, stderr := c.runCmdInContainer(id, cmd)
+
+	// Verify the file arrived and is non-empty
+	verifyCmd := []string{"test", "-s", dest}
+	_, verifyErr := c.runCmdInContainer(id, verifyCmd)
+	if len(verifyErr) > 0 {
+		msg := "download failed: " + src
+		if len(stderr) > 0 {
+			msg += "\nwget output: " + string(stderr)
+		}
+		panic(msg)
 	}
-
-	c.runCmdInContainer(id, cmd)
 }
 
 func (c Controller) runCmdInContainer(id string, cmd []string) ([]byte, []byte) {

internal/container/controller_test.go

@@ -54,7 +54,11 @@ func TestController_EnsureImage(t *testing.T) {
 	}))
 	defer ts.Close()
 
-	c.DownloadFile(id, ts.URL, "test.txt")
+	// httptest binds to 127.0.0.1 which is unreachable from inside the
+	// container, so DownloadFile now correctly panics on the failed download.
+	assert.Panics(t, func() {
+		c.DownloadFile(id, ts.URL+"/test.bak", "/tmp")
+	})
 
 	err = c.ContainerStop(id)
 	checkErr(err)