fix: instruction prefixes with multiline instructions

Author: jimschubert

src/main/java/com/github/jimschubert/rewrite/docker/internal/DockerfileParser.java

@@ -687,16 +687,6 @@ public Docker.Document parse(InputStream input) {
                     continue;
                 }
 
-                if (scanner.hasNext() && !eof.isEmpty()) {
-                    // at this point, we have a line, so eof is the line prefix
-                    state.appendPrefix(eof);
-                    eof = Space.EMPTY;
-                } else if (!scanner.hasNext()) {
-                    // if we are at the end of the file with a newline
-                    eof = Space.append(eof, eol);
-                    eol = Space.EMPTY;
-                }
-
                 line = handleRightPadding(line, state);
 
                 // TODO: consider a better way to handle "inline" comments
@@ -718,12 +708,26 @@ public Docker.Document parse(InputStream input) {
                 }
 
                 parser.append(line);
-                if (line.endsWith(state.getEscapeString())) {
+                // TODO: should we throw an error here if the line ends in the escape char and there are no more lines?
+                if (scanner.hasNext() && line.endsWith(state.getEscapeString())) {
                     parser.append(eol.getWhitespace());
                     state.isContinuation(true);
                     continue;
                 }
 
+                if (!eof.isEmpty() && (state.isContinuation() || scanner.hasNext())) {
+                    // any previously gathered whitespace is the prefix to this instruction
+                    state.appendPrefix(eof);
+                    eof = Space.EMPTY;
+                }
+
+                 if (!scanner.hasNext()) {
+                    // if we are at the end of the file with a newline, that is our eof.
+                    // other conditions such as multiple newlines or whitespace are handled earlier
+                    eof = Space.append(eof, eol);
+                    eol = Space.EMPTY;
+                }
+
                 Docker.Instruction instr = parser.parse();
                 if (instr != null) {
                     instr = instr.withEol(eol);

src/test/java/com/github/jimschubert/rewrite/docker/internal/DockerfileParserTest.java

@@ -1366,4 +1366,35 @@ void testDockerfileWithCommentBetweenContinuationLines() {
         assertLiteral(args.get(10), Quoting.UNQUOTED, " ", "World", "");
 
     }
+
+    @Test
+    void testInstructionPrefixWithContinuationCommands() {
+        String monster = """
+        ENV LAST_COMMIT=e2db71ff940a8b8c08c4ae894b952bfe7f0cf309
+        
+        RUN set -eux; \\
+        	\\
+        	apk add --no-cache --virtual .build-deps
+        """;
+
+        DockerfileParser parser = new DockerfileParser();
+        Docker.Document doc = parser.parse(new ByteArrayInputStream(monster.getBytes(StandardCharsets.UTF_8)));
+
+        Docker.Stage stage = assertSingleStageWithChildCount(doc, 2);
+        Docker.Env env = (Docker.Env) stage.getChildren().get(0);
+        assertEquals(Space.EMPTY, env.getPrefix());
+        assertEquals(1, env.getArgs().size());
+        assertRightPaddedArg(env.getArgs().get(0), Quoting.UNQUOTED, " ", "LAST_COMMIT", true, "e2db71ff940a8b8c08c4ae894b952bfe7f0cf309", "");
+
+        Docker.Run run = (Docker.Run) stage.getChildren().get(1);
+        assertEquals(Space.NEWLINE, run.getPrefix());
+        assertEquals(7, run.getCommands().size());
+        assertLiteral(run.getCommands().get(0), Quoting.UNQUOTED, " ", "set", "");
+        assertLiteral(run.getCommands().get(1), Quoting.UNQUOTED, " ", "-eux;", " \\\n\t\\\n\t");
+        assertLiteral(run.getCommands().get(2), Quoting.UNQUOTED, "", "apk", "");
+        assertLiteral(run.getCommands().get(3), Quoting.UNQUOTED, " ", "add", "");
+        assertLiteral(run.getCommands().get(4), Quoting.UNQUOTED, " ", "--no-cache", "");
+        assertLiteral(run.getCommands().get(5), Quoting.UNQUOTED, " ", "--virtual", "");
+        assertLiteral(run.getCommands().get(6), Quoting.UNQUOTED, " ", ".build-deps", "");
+    }
 }