Merge #11687: External wallet files

be8ab7d08 Create new wallet databases as directories rather than files (Russell Yanofsky)
26c06f24e Allow wallet files not in -walletdir directory (Russell Yanofsky)
d8a99f65e Allow wallet files in multiple directories (Russell Yanofsky)

Pull request description:

  This change consists of three commits:

  * The first commit is a pure refactoring that removes the restriction that two wallets can only be opened at the same time if they are contained in the same directory.
  * The second commit removes the restriction that `-wallet` filenames can only refer to files in the `-walletdir` directory.
  * The third commit makes second commit a little safer by changing bitcoin to create wallet databases as directories rather than files, so they can be safely backed up.

  All three commits should be straightforward:

  *  The first commit adds around 20 lines of new code and then updates a bunch of function signatures (generally updating them to take plain fs::path parameters, instead of combinations of strings, fs::paths, and objects like CDBEnv and CWalletDBWrapper).
  * The second commit removes two `-wallet` filename checks and adds some test cases to the multiwallet unit test.
  * The third commit just changes the mapping from specified wallet paths to bdb environment & data paths.

    ---

  **Note:** For anybody looking at this PR for the first time, I think you can skip the comments before _20 Nov_ and start reading at https://github.com/bitcoin/bitcoin/pull/11687#issuecomment-345625565. Comments before  _20 Nov_ were about an earlier version of the PR that didn't include the third commit, and then confusion from not seeing the first commit.

Tree-SHA512: 00bbb120fe0df847cf57014f75f1f7f1f58b0b62fa0b3adab4560163ebdfe06ccdfff33b4231693f03c5dc23601cb41954a07bcea9a4919c8d42f7d62bcf6024

doc/release-notes.md

@@ -63,6 +63,36 @@ RPC changes
 
 - The `fundrawtransaction` rpc will reject the previously deprecated `reserveChangeKey` option.
 
+External wallet files
+---------------------
+
+The `-wallet=<path>` option now accepts full paths instead of requiring wallets
+to be located in the -walletdir directory.
+
+Newly created wallet format
+---------------------------
+
+If `-wallet=<path>` is specified with a path that does not exist, it will now
+create a wallet directory at the specified location (containing a wallet.dat
+data file, a db.log file, and database/log.?????????? files) instead of just
+creating a data file at the path and storing log files in the parent
+directory. This should make backing up wallets more straightforward than
+before because the specified wallet path can just be directly archived without
+having to look in the parent directory for transaction log files.
+
+For backwards compatibility, wallet paths that are names of existing data files
+in the `-walletdir` directory will continue to be accepted and interpreted the
+same as before.
+
+Low-level RPC changes
+---------------------
+
+- When bitcoin is not started with any `-wallet=<path>` options, the name of
+  the default wallet returned by `getwalletinfo` and `listwallets` RPCs is
+  now the empty string `""` instead of `"wallet.dat"`. If bitcoin is started
+  with any `-wallet=<path>` options, there is no change in behavior, and the
+  name of any wallet is just its `<path>` string.
+
 Credits
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