Don't allow relative -walletdir paths

Also warn if bitcoind is configured to use a relative -datadir path.

Specifying paths relative to the current working directory in a daemon process
can be dangerous, because files can fail to be located even if the
configuration doesn't change, but the daemon is started up differently.

Specifying a relative -datadir now adds a warning to the debug log. It would
not be backwards-compatible to forbid relative -datadir paths entirely, and it
could also be also inconvenient for command line testing.

Specifying a relative -walletdir now results in a startup error. But since the
-walletdir option is new in 0.16.0, there should be no compatibility issues.
Another reason not to use working directory paths for -walletdir specifically
is that the default -walletdir is a "wallets" subdirectory inside the datadir,
so it could be surprising that setting -walletdir manually would choose a
directory rooted in a completely different location.

7 files changed, 32 insertions, 16 deletions

diff --git a/doc/release-notes.md b/doc/release-notes.md
index ecca08d96d..29377d4a1d 100644
--- a/doc/release-notes.md
+++ b/doc/release-notes.md
@@ -90,9 +90,8 @@ bitcoin data directory. The behavior is now:
   already exists in the data directory root, then wallets will be stored in the
   `wallets/` subdirectory by default.
 - The location of the wallets directory can be overridden by specifying a
-  `-walletdir=<path>` option where `<path>` can be an absolute path or a
-  relative path (relative to the current working directory). `<path>` can
-  also be a path to symlink to a directory.
+  `-walletdir=<path>` option where `<path>` can be an absolute path to a
+  directory or directory symlink.
 
 Care should be taken when choosing the wallets directory location, as if it
 becomes unavailable during operation, funds may be lost.
diff --git a/src/init.cpp b/src/init.cpp
index b48802637b..d1d733af94 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -1210,6 +1210,15 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
     LogPrintf("Using config file %s\n", GetConfigFile(gArgs.GetArg("-conf", BITCOIN_CONF_FILENAME)).string());
     LogPrintf("Using at most %i automatic connections (%i file descriptors available)\n", nMaxConnections, nFD);
 
+    // Warn about relative -datadir path.
+    if (gArgs.IsArgSet("-datadir") && !fs::path(gArgs.GetArg("-datadir", "")).is_absolute()) {
+        LogPrintf("Warning: relative datadir option '%s' specified, which will be interpreted relative to the "
+                  "current working directory '%s'. This is fragile, because if bitcoin is started in the future "
+                  "from a different location, it will be unable to locate the current data files. There could "
+                  "also be data loss if bitcoin is started while in a temporary directory.\n",
+            gArgs.GetArg("-datadir", ""), fs::current_path().string());
+    }
+
     InitSignatureCache();
     InitScriptExecutionCache();
 
diff --git a/src/wallet/init.cpp b/src/wallet/init.cpp
index 0f453f154f..2d26f7ae0f 100644
--- a/src/wallet/init.cpp
+++ b/src/wallet/init.cpp
@@ -205,11 +205,15 @@ bool VerifyWallets()
         return true;
     }
 
-    if (gArgs.IsArgSet("-walletdir") && !fs::is_directory(GetWalletDir())) {
-        if (fs::exists(fs::system_complete(gArgs.GetArg("-walletdir", "")))) {
-            return InitError(strprintf(_("Specified -walletdir \"%s\" is not a directory"), gArgs.GetArg("-walletdir", "").c_str()));
+    if (gArgs.IsArgSet("-walletdir")) {
+        fs::path wallet_dir = gArgs.GetArg("-walletdir", "");
+        if (!fs::exists(wallet_dir)) {
+            return InitError(strprintf(_("Specified -walletdir \"%s\" does not exist"), wallet_dir.string()));
+        } else if (!fs::is_directory(wallet_dir)) {
+            return InitError(strprintf(_("Specified -walletdir \"%s\" is not a directory"), wallet_dir.string()));
+        } else if (!wallet_dir.is_absolute()) {
+            return InitError(strprintf(_("Specified -walletdir \"%s\" is a relative path"), wallet_dir.string()));
         }
-        return InitError(strprintf(_("Specified -walletdir \"%s\" does not exist"), gArgs.GetArg("-walletdir", "").c_str()));
     }
 
     LogPrintf("Using wallet directory %s\n", GetWalletDir().string());
diff --git a/src/wallet/walletutil.cpp b/src/wallet/walletutil.cpp
index f15e5de1e2..7c97b668ae 100644
--- a/src/wallet/walletutil.cpp
+++ b/src/wallet/walletutil.cpp
@@ -9,7 +9,7 @@ fs::path GetWalletDir()
     fs::path path;
 
     if (gArgs.IsArgSet("-walletdir")) {
-        path = fs::system_complete(gArgs.GetArg("-walletdir", ""));
+        path = gArgs.GetArg("-walletdir", "");
         if (!fs::is_directory(path)) {
             // If the path specified doesn't exist, we return the deliberately
             // invalid empty string.
diff --git a/test/functional/multiwallet.py b/test/functional/multiwallet.py
index 0891829127..b07e451667 100755
--- a/test/functional/multiwallet.py
+++ b/test/functional/multiwallet.py
@@ -30,6 +30,10 @@ class MultiWalletTest(BitcoinTestFramework):
 
         self.stop_nodes()
 
+        self.assert_start_raises_init_error(0, ['-walletdir=wallets'], 'Error: Specified -walletdir "wallets" does not exist')
+        self.assert_start_raises_init_error(0, ['-walletdir=wallets'], 'Error: Specified -walletdir "wallets" is a relative path', cwd=data_dir())
+        self.assert_start_raises_init_error(0, ['-walletdir=debug.log'], 'Error: Specified -walletdir "debug.log" is not a directory', cwd=data_dir())
+
         # should not initialize if there are duplicate wallets
         self.assert_start_raises_init_error(0, ['-wallet=w1', '-wallet=w1'], 'Error loading wallet w1. Duplicate -wallet filename specified.')
 
diff --git a/test/functional/test_framework/test_framework.py b/test/functional/test_framework/test_framework.py
index 5d4f8e6720..f8d66def64 100755
--- a/test/functional/test_framework/test_framework.py
+++ b/test/functional/test_framework/test_framework.py
@@ -220,18 +220,18 @@ class BitcoinTestFramework():
         for i in range(num_nodes):
             self.nodes.append(TestNode(i, self.options.tmpdir, extra_args[i], rpchost, timewait=timewait, binary=binary[i], stderr=None, mocktime=self.mocktime, coverage_dir=self.options.coveragedir, use_cli=self.options.usecli))
 
-    def start_node(self, i, extra_args=None, stderr=None):
+    def start_node(self, i, *args, **kwargs):
         """Start a bitcoind"""
 
         node = self.nodes[i]
 
-        node.start(extra_args, stderr)
+        node.start(*args, **kwargs)
         node.wait_for_rpc_connection()
 
         if self.options.coveragedir is not None:
             coverage.write_all_rpc_commands(self.options.coveragedir, node.rpc)
 
-    def start_nodes(self, extra_args=None):
+    def start_nodes(self, extra_args=None, *args, **kwargs):
         """Start multiple bitcoinds"""
 
         if extra_args is None:
@@ -239,7 +239,7 @@ class BitcoinTestFramework():
         assert_equal(len(extra_args), self.num_nodes)
         try:
             for i, node in enumerate(self.nodes):
-                node.start(extra_args[i])
+                node.start(extra_args[i], *args, **kwargs)
             for node in self.nodes:
                 node.wait_for_rpc_connection()
         except:
@@ -271,10 +271,10 @@ class BitcoinTestFramework():
         self.stop_node(i)
         self.start_node(i, extra_args)
 
-    def assert_start_raises_init_error(self, i, extra_args=None, expected_msg=None):
+    def assert_start_raises_init_error(self, i, extra_args=None, expected_msg=None, *args, **kwargs):
         with tempfile.SpooledTemporaryFile(max_size=2**16) as log_stderr:
             try:
-                self.start_node(i, extra_args, stderr=log_stderr)
+                self.start_node(i, extra_args, stderr=log_stderr, *args, **kwargs)
                 self.stop_node(i)
             except Exception as e:
                 assert 'bitcoind exited' in str(e)  # node must have shutdown
diff --git a/test/functional/test_framework/test_node.py b/test/functional/test_framework/test_node.py
index 589a8f3969..7c784d8840 100755
--- a/test/functional/test_framework/test_node.py
+++ b/test/functional/test_framework/test_node.py
@@ -81,13 +81,13 @@ class TestNode():
             assert self.rpc_connected and self.rpc is not None, "Error: no RPC connection"
             return getattr(self.rpc, name)
 
-    def start(self, extra_args=None, stderr=None):
+    def start(self, extra_args=None, stderr=None, *args, **kwargs):
         """Start the node."""
         if extra_args is None:
             extra_args = self.extra_args
         if stderr is None:
             stderr = self.stderr
-        self.process = subprocess.Popen(self.args + extra_args, stderr=stderr)
+        self.process = subprocess.Popen(self.args + extra_args, stderr=stderr, *args, **kwargs)
         self.running = True
         self.log.debug("bitcoind started, waiting for RPC to come up")