Merge bitcoin/bitcoin#25106: rpc: dumptxoutset: check fopen return code

9feb887082be911a8342f8090af4dca3db76db9b rpc: check `fopen` return code in dumptxoutset (Sebastian Falbesoner)

Pull request description:

  This change improves the usability of the `dumptxoutset` RPC in two ways, in the case that an invalid path is passed:
  1. return from the RPC immediately, rather then when the file is first tried to be written (which is _after_ calculating the UTXO set hash)
  2. return a proper return code and error message instead of the cryptic message that appears on master currently (see below)

  master branch:
  (error message appears after several minutes on my machine)
  ```
  $ ./src/bitcoin-cli dumptxoutset /invalid/path
  error code: -1
  error message:
  CAutoFile::operator<<: file handle is nullptr: unspecified iostream_category error
  ```

  PR branch:
  (error message appears immediately)
  ```
  $ ./src/bitcoin-cli dumptxoutset /invalid/path
  error code: -8
  error message:
  Couldn't open file /invalid/path.incomplete for writing.
  ```

ACKs for top commit:
  w0xlt:
    Code Review ACK 9feb887082

Tree-SHA512: e8695a7e86f26cc3b086d6bc6888388061f1dee439f76409b3ee11d35032bfd9cfa5349b728cd7f45bcffd999ecf9a6a991be172ce587b9b14503d9916b6e984

src/rpc/blockchain.cpp

@@ -2281,6 +2281,12 @@ static RPCHelpMan dumptxoutset()
 
     FILE* file{fsbridge::fopen(temppath, "wb")};
     CAutoFile afile{file, SER_DISK, CLIENT_VERSION};
+    if (afile.IsNull()) {
+        throw JSONRPCError(
+            RPC_INVALID_PARAMETER,
+            "Couldn't open file " + temppath.u8string() + " for writing.");
+    }
+
     NodeContext& node = EnsureAnyNodeContext(request.context);
     UniValue result = CreateUTXOSnapshot(
         node, node.chainman->ActiveChainstate(), afile, path, temppath);

test/functional/rpc_dumptxoutset.py

@@ -49,9 +49,13 @@ class DumptxoutsetTest(BitcoinTestFramework):
             out['txoutset_hash'], '1f7e3befd45dc13ae198dfbb22869a9c5c4196f8e9ef9735831af1288033f890')
         assert_equal(out['nchaintx'], 101)
 
-        # Specifying a path to an existing file will fail.
+        # Specifying a path to an existing or invalid file will fail.
         assert_raises_rpc_error(
             -8, '{} already exists'.format(FILENAME),  node.dumptxoutset, FILENAME)
+        invalid_path = str(Path(node.datadir) / "invalid" / "path")
+        assert_raises_rpc_error(
+            -8, "Couldn't open file {}.incomplete for writing".format(invalid_path), node.dumptxoutset, invalid_path)
+
 
 if __name__ == '__main__':
     DumptxoutsetTest().main()