gguf-py : always defer GGUFWrite output file opening

Changing what happens when the output file is opened will be easier,
since this reduces the cases to consider.

* gguf-py : prevent GGUFWriter from writing all tensors multiple times

It was already checked with an assertion before, but using WriterState
should make the error message slightly less cryptic.

gguf-py/gguf/gguf_writer.py

@@ -51,10 +51,12 @@ class WriterState(Enum):
     HEADER  = auto()
     KV_DATA = auto()
     TI_DATA = auto()
+    WEIGHTS = auto()
 
 
 class GGUFWriter:
     fout: BufferedWriter | None
+    path: os.PathLike[str] | str | None
     temp_file: tempfile.SpooledTemporaryFile[bytes] | None
     tensors: dict[str, TensorInfo]
     kv_data: dict[str, GGUFValue]
@@ -77,7 +79,8 @@ class GGUFWriter:
         self, path: os.PathLike[str] | str | None, arch: str, use_temp_file: bool = False,
         endianess: GGUFEndian = GGUFEndian.LITTLE,
     ):
-        self.fout = open(path, "wb") if path is not None else None
+        self.fout = None
+        self.path = path
         self.arch = arch
         self.endianess = endianess
         self.data_alignment = GGUF_DEFAULT_ALIGNMENT
@@ -88,19 +91,29 @@ class GGUFWriter:
         logger.info("gguf: This GGUF file is for {0} Endian only".format(
             "Big" if self.endianess == GGUFEndian.BIG else "Little",
         ))
-        self.state = WriterState.NO_FILE if self.fout is None else WriterState.EMPTY
+        self.state = WriterState.NO_FILE
 
         self.add_architecture()
 
-    def write_header_to_file(self, path: os.PathLike[str] | str | None = None) -> None:
-        # NOTE: not checking for WriterState.NO_FILE,
-        # because writing can technically be started over from any state,
-        # as long as a new path is provided
+    def open_output_file(self, path: os.PathLike[str] | str | None = None) -> None:
+        if self.state is WriterState.EMPTY and self.fout is not None and (path is None or path == self.path):
+            # allow calling this multiple times as long as the path is the same
+            return
+        if self.state is not WriterState.NO_FILE:
+            raise ValueError(f'Expected output file to be not yet opened, got {self.state}')
+
         if path is not None:
+            self.path = path
+
+        if self.path is not None:
             if self.fout is not None:
                 self.fout.close()
-            self.fout = open(path, "wb")
+            self.fout = open(self.path, "wb")
             self.state = WriterState.EMPTY
+
+    def write_header_to_file(self, path: os.PathLike[str] | str | None = None) -> None:
+        self.open_output_file(path)
+
         if self.state is not WriterState.EMPTY:
             raise ValueError(f'Expected output file to be empty, got {self.state}')
 
@@ -206,8 +219,8 @@ class GGUFWriter:
         self, name: str, tensor_shape: Sequence[int], tensor_dtype: np.dtype,
         tensor_nbytes: int, raw_dtype: GGMLQuantizationType | None = None,
     ) -> None:
-        if self.state is not WriterState.EMPTY and self.state is not WriterState.NO_FILE:
-            raise ValueError(f'Expected output file to be empty or absent, got {self.state}')
+        if self.state is not WriterState.NO_FILE:
+            raise ValueError(f'Expected output file to be not yet opened, got {self.state}')
 
         if name in self.tensors:
             raise ValueError(f'Duplicated tensor name {name!r}')
@@ -263,8 +276,8 @@ class GGUFWriter:
             fp.write(bytes([0] * pad))
 
     def write_tensor_data(self, tensor: np.ndarray[Any, Any]) -> None:
-        if self.state is not WriterState.TI_DATA:
-            raise ValueError(f'Expected output file to contain tensor info, got {self.state}')
+        if self.state is not WriterState.TI_DATA and self.state is not WriterState.WEIGHTS:
+            raise ValueError(f'Expected output file to contain tensor info or weights, got {self.state}')
         assert self.fout is not None
 
         if self.endianess == GGUFEndian.BIG:
@@ -273,6 +286,8 @@ class GGUFWriter:
         tensor.tofile(self.fout)
         self.write_padding(self.fout, tensor.nbytes)
 
+        self.state = WriterState.WEIGHTS
+
     def write_tensors_to_file(self, *, progress: bool = False) -> None:
         self.write_ti_data_to_file()
 
@@ -299,14 +314,14 @@ class GGUFWriter:
                     bar.update(ti.nbytes)
                 self.write_padding(self.fout, ti.nbytes)
                 ti.tensor = None
+        else:
+            self.temp_file.seek(0)
 
-            return
+            shutil.copyfileobj(self.temp_file, self.fout)
+            self.flush()
+            self.temp_file.close()
 
-        self.temp_file.seek(0)
-
-        shutil.copyfileobj(self.temp_file, self.fout)
-        self.flush()
-        self.temp_file.close()
+        self.state = WriterState.WEIGHTS
 
     def flush(self) -> None:
         assert self.fout is not None