Raise exception if resulting PDF might appear blank in a known in some PDF viewers

Fixes #1187

src/ocrmypdf/_exec/ghostscript.py

@@ -17,7 +17,7 @@ from subprocess import PIPE, CalledProcessError
 from packaging.version import Version
 from PIL import Image, UnidentifiedImageError
 
-from ocrmypdf.exceptions import SubprocessOutputError
+from ocrmypdf.exceptions import ColorConversionNeededError, SubprocessOutputError
 from ocrmypdf.helpers import Resolution
 from ocrmypdf.subprocess import get_version, run, run_polling_stderr
 
@@ -64,10 +64,6 @@ class DuplicateFilter(logging.Filter):
 log.addFilter(DuplicateFilter(log))
 
 
-# Ghostscript executable - gswin32c is not supported
-GS = 'gswin64c' if os.name == 'nt' else 'gs'
-
-
 def version() -> Version:
     return Version(get_version(GS))
 
@@ -77,6 +73,15 @@ def _gs_error_reported(stream) -> bool:
     return bool(match)
 
 
+def _gs_devicen_reported(stream) -> bool:
+    match = re.search(
+        r'DeviceN.*inappropriate alternate',
+        stream,
+        flags=re.IGNORECASE | re.MULTILINE,
+    )
+    return bool(match)
+
+
 def rasterize_pdf(
     input_file: os.PathLike,
     output_file: os.PathLike,
@@ -250,7 +255,6 @@ def generate_pdfa(
         ]
     )
     args_gs.extend(fspath(s) for s in pdf_pages)  # Stringify Path objs
-
     try:
         with Path(output_file).open('wb') as output:
             p = run_polling_stderr(
@@ -279,3 +283,5 @@ def generate_pdfa(
             # the **** pattern to split the stderr into parts.
             for part in stderr.split('****'):
                 log.error(part)
+        if _gs_devicen_reported(stderr):
+            raise ColorConversionNeededError()

src/ocrmypdf/exceptions.py

@@ -137,3 +137,16 @@ class TaggedPDFError(InputFileError):
         override this error.
         """
     )
+
+
+class ColorConversionNeededError(BadArgsError):
+    """PDF needs color conversion."""
+
+    message = dedent(
+        """\
+        The input PDF has an unusual color space. Use
+        --color-conversion-strategy to convert to a common color space
+        such as RGB, or use --output-type pdf to skip PDF/A conversion
+        and retain the original color space.
+        """
+    )