Fix virtual package parsing to prevent invalid cache entries

.gitignore

@@ -1,2 +1,3 @@
 src/cmd/apt_query/apt_query*
+apt_query_test*
 *.log

src/cmd/apt_query/main_test.go

@@ -68,3 +68,9 @@ func TestNormalizedList_VirtualPackagesExists_StdoutsConcretePackage(t *testing.
 	result := cmdtesting.New(t, createReplayLogs).Run("normalized-list", "libvips")
 	result.ExpectSuccessfulOut("libvips42=8.9.1-2")
 }
+
+func TestNormalizedList_VirtualPackageWithNoProviders_StderrsErrorMessage(t *testing.T) {
+	var result = cmdtesting.New(t, createReplayLogs).Run("normalized-list", "python")
+	result.ExpectError(`Encountered error resolving some or all package names, see combined std[out,err] below.
+virtual package 'python' has no concrete package providers available`)
+}

src/cmd/apt_query/testlogs/testnormalizedlist_virtualpackagewithnoproviders_stderrserrormessage.log

@@ -0,0 +1,17 @@
+2025/10/04 06:59:56 Debug log created at /home/runner/work/cache-apt-pkgs-action/cache-apt-pkgs-action/src/cmd/apt_query/apt_query.log
+2025/10/04 07:00:00 EXECUTION-OBJ-START
+{
+ "Cmd": "apt-cache --quiet=0 --no-all-versions show python",
+ "CombinedOut": "N: Can't select candidate version from package python as it has no candidate\nN: Can't select versions from package 'python' as it is purely virtual\nN: No packages found\n",
+ "ExitCode": 0
+}
+EXECUTION-OBJ-END
+2025/10/04 07:00:00 EXECUTION-OBJ-START
+{
+ "Cmd": "bash -c apt-cache showpkg python | grep -A 1 \"Reverse Provides\" | tail -1",
+ "CombinedOut": "Reverse Provides: \n",
+ "ExitCode": 0
+}
+EXECUTION-OBJ-END
+2025/10/04 07:00:00 Encountered error resolving some or all package names, see combined std[out,err] below.
+virtual package 'python' has no concrete package providers available

src/internal/common/apt.go

@@ -31,7 +31,13 @@ func isErrLine(line string) bool {
 	return strings.HasPrefix(line, "E: ") || strings.HasPrefix(line, "N: ")
 }
 
+// isVirtualPackage checks if a string indicates a purely virtual package message.
+func isVirtualPackage(message string) bool {
+	return strings.Contains(message, "as it is purely virtual")
+}
+
 // Resolves virtual packages names to their concrete one.
+// Returns the first available concrete package provider, or an error if none exist.
 func getNonVirtualPackage(executor exec.Executor, name string) (pkg *AptPackage, err error) {
 	execution := executor.Exec("bash", "-c", fmt.Sprintf("apt-cache showpkg %s | grep -A 1 \"Reverse Provides\" | tail -1", name))
 	err = execution.Error()
@@ -42,10 +48,19 @@ func getNonVirtualPackage(executor exec.Executor, name string) (pkg *AptPackage,
 	if isErrLine(execution.CombinedOut) {
 		return pkg, execution.Error()
 	}
+
+	// Check if the output is just "Reverse Provides:" with no actual package info
+	trimmedOutput := strings.TrimSpace(execution.CombinedOut)
+	if trimmedOutput == "Reverse Provides:" {
+		return pkg, fmt.Errorf("virtual package '%s' has no concrete package providers available", name)
+	}
+
 	splitLine := GetSplitLine(execution.CombinedOut, " ", 3)
 	if len(splitLine.Words) < 2 {
 		return pkg, fmt.Errorf("unable to parse space delimited line's package name and version from apt-cache showpkg output below:\n%s", execution.CombinedOut)
 	}
+
+	// Successfully found a concrete package provider
 	return &AptPackage{Name: splitLine.Words[0], Version: splitLine.Words[1]}, nil
 }
 
@@ -67,7 +82,7 @@ func getPackage(executor exec.Executor, paragraph string) (pkg *AptPackage, err
 
 		case "N":
 			// e.g.  Can't select versions from package 'libvips' as it is purely virtual
-			if strings.Contains(splitLine.Words[1], "as it is purely virtual") {
+			if isVirtualPackage(splitLine.Words[1]) {
 				return getNonVirtualPackage(executor, GetSplitLine(splitLine.Words[1], "'", 4).Words[2])
 			}
 			if strings.HasPrefix(splitLine.Words[1], "Unable to locate package") && !ArrContainsString(errMsgs, splitLine.Line) {
@@ -112,10 +127,6 @@ func GetAptPackages(executor exec.Executor, names []string) (AptPackages, error)
 		}
 	}
 
-	if len(errMsgs) > 0 {
-		errMsgs = append(errMsgs, strings.Join(errMsgs, "\n"))
-	}
-
 	sort.Slice(pkgs, func(i, j int) bool {
 		return pkgs[i].Name < pkgs[j].Name
 	})