datastore: plugin to replicate datastore commands before next release.

See https://github.com/ElementsProject/lightning/pull/4674
(I manually tested that it passes all the tests there, too!)

When they finally upgrade the node, this automatically puts any
datastore data into the inbuilt datastore and shuts down.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

datastore/datastore-plugin.py

@@ -0,0 +1,91 @@
+#!/usr/bin/env python3
+"""This does the actual datastore work, if the main plugin says there's no
+datastore support.  We can't even load this if there's real datastore support.
+"""
+from pyln.client import Plugin, RpcError
+import shelve
+
+
+plugin = Plugin()
+
+
+def datastore_entry(key, data):
+    """Return a dict representing the entry"""
+
+    ret = {'key': key, 'hex': data.hex()}
+
+    # FFS, Python3 seems happy with \0 in UTF-8.
+    if 0 not in data:
+        try:
+            ret['string'] = data.decode('utf8')
+        except UnicodeDecodeError:
+            pass
+    return ret
+
+
+@plugin.method("datastore")
+def datastore(plugin, key, string=None, hex=None, mode="must-create"):
+    """Add a {key} and {hex}/{string} data to the data store"""
+
+    if string is not None:
+        if hex is not None:
+            raise RpcError("datastore", {'key': key},
+                           {'message': "Cannot specify both string or hex"})
+        data = bytes(string, encoding="utf8")
+    elif hex is None:
+        raise RpcError("datastore", {'key': key},
+                       {'message': "Must specify string or hex"})
+    else:
+        data = bytes.fromhex(hex)
+
+    print("key={}, data={}, mode={}".format(key, data, mode))
+    if mode == "must-create":
+        if key in plugin.datastore:
+            raise RpcError("datastore", {'key': key},
+                           {'message': "already exists"})
+    elif mode == "must-replace":
+        if key not in plugin.datastore:
+            raise RpcError("datastore", {'key': key},
+                           {'message': "does not exist"})
+    elif mode == "create-or-replace":
+        pass
+    elif mode == "must-append":
+        if key not in plugin.datastore:
+            raise RpcError("datastore", {'key': key},
+                           {'message': "does not exist"})
+        data = plugin.datastore[key] + data
+    elif mode == "create-or-append":
+        data = plugin.datastore.get(key, bytes()) + data
+    else:
+        raise RpcError("datastore", {'key': key}, {'message': "invalid mode"})
+
+    plugin.datastore[key] = data
+    return datastore_entry(key, data)
+
+
+@plugin.method("deldatastore")
+def deldatastore(plugin, key):
+    """Remove a {key} from the data store"""
+
+    ret = datastore_entry(key, plugin.datastore[key])
+    del plugin.datastore[key]
+    return ret
+
+
+@plugin.method("listdatastore")
+def listdatastore(plugin, key=None):
+    """List datastore entries"""
+    if key is None:
+        return {'datastore': [datastore_entry(k, d)
+                              for k, d in plugin.datastore.items()]}
+    if key in plugin.datastore:
+        return {'datastore': [datastore_entry(key, plugin.datastore[key])]}
+    return {'datastore': []}
+
+
+@plugin.init()
+def init(options, configuration, plugin):
+    plugin.datastore = shelve.open('datastore.dat')
+
+
+plugin.run()