"block" instruction implemented

The "block" instruction allows mulitple instructions to be grouped in
order to be executed based on a single condition, in a common loop, with
a local variable scope. In addition, it provides a way to recover from
errors.

README.md

@@ -106,6 +106,8 @@ skipping.
   |  |  |  |  |--vm01
   |  |  |  |  |--vm05
   |  |  |  |  |--vm06
+  |--@reedmably_evil:
+  |  |--evil-but-nicer-vm
   |--@ungrouped:
   |  |--evil-vm
   |  |--localhost

example/00-data.yml

@@ -6,6 +6,12 @@ all:
     # All of this should obviously come from some other inventory plugin.
 
     evil-vm:
+      inv__data:
+        network: death
+        service: evil
+        instance: chaos
+        unredeemable: true
+    evil-but-nicer-vm:
       inv__data:
         network: death
         service: evil

example/01-test-reconstructed.yml

@@ -14,18 +14,33 @@ instructions:
     action: stop
 
   # Fail when the host name starts with "evil".
-  - when: inventory_hostname.startswith( 'evil' )
-    action: fail
-    msg: "{{ inventory_hostname }} is obviously evil, skipping."
+  - action: block
+    block:
+      - when: inventory_hostname.startswith( 'evil' )
+        action: fail
+        msg: "{{ inventory_hostname }} is obviously evil, skipping."
+    rescue:
+      # Do not crash on redeemably evil VMs, but still skip them.
+      - when: inv__data.unredeemable is defined
+        action: fail
+        msg: "{{ reconstructed_error }}"
+      - action: create_group
+        group: reedmably_evil
+      - action: add_host
+        group: reedmably_evil
+      - action: stop
 
   # Only create the managed groups if we *have* managed hosts
-  - loop: [managed, by_environment, by_network, by_failover_stack, by_service]
-    action: create_group
-    group: "{{ item }}"
-  - loop: [by_environment, by_network, by_failover_stack, by_service]
-    action: add_child
+  - action: create_group
     group: managed
-    child: "{{ item }}"
+  - loop: [by_environment, by_network, by_failover_stack, by_service]
+    action: block
+    block:
+      - action: create_group
+        group: "{{ item }}"
+      - action: add_child
+        group: managed
+        child: "{{ item }}"
 
   # Copy inv__data fields to separate inv__ variables
   - loop:
@@ -98,32 +113,28 @@ instructions:
 
   # Component group. We add the host directly if there is no subcomponent.
   - when: inv__component is defined
-    action: set_var
-    name: comp_group
-    value: "svcm_{{ inv__service }}_{{ inv__component }}"
-  - when: inv__component is defined
-    action: create_group
-    group: "{{ comp_group }}"
-  - when: inv__component is defined
-    action: add_child
-    group: "{{ service_group }}"
-    child: "{{ comp_group }}"
-  - when: inv__component is defined and inv__subcomponent is not defined
-    action: add_host
-    group: "{{ comp_group }}"
-
-  # Subcomponent group.
-  - when: inv__component is defined and inv__subcomponent is defined
-    action: set_var
-    name: subcomp_group
-    value: "svcm_{{ inv__service }}_{{ inv__subcomponent }}"
-  - when: inv__component is defined and inv__subcomponent is defined
-    action: create_group
-    group: "{{ subcomp_group }}"
-  - when: inv__component is defined and inv__subcomponent is defined
-    action: add_child
-    group: "{{ comp_group }}"
-    child: "{{ subcomp_group }}"
-  - when: inv__component is defined and inv__subcomponent is defined
-    action: add_host
-    group: "{{ subcomp_group }}"
+    action: block
+    locals:
+      comp_group: "svcm_{{ inv__service }}_{{ inv__component }}"
+    block:
+    - action: create_group
+      group: "{{ comp_group }}"
+    - action: add_child
+      group: "{{ service_group }}"
+      child: "{{ comp_group }}"
+    # Subcomponent group, or lack thereof.
+    - when: inv__subcomponent is not defined
+      action: add_host
+      group: "{{ comp_group }}"
+    - when: inv__subcomponent is defined
+      action: block
+      locals:
+        subcomp_group: "svcm_{{ inv__service }}_{{ inv__subcomponent }}"
+      block:
+      - action: create_group
+        group: "{{ subcomp_group }}"
+      - action: add_child
+        group: "{{ comp_group }}"
+        child: "{{ subcomp_group }}"
+      - action: add_host
+        group: "{{ subcomp_group }}"

inventory_plugins/reconstructed.py

@@ -37,6 +37,16 @@ DOCUMENTATION = """
           representing a condition which will be checked before the instruction
           is executed.
         - The C(action) field must be set to one of the following values.
+        - The C(block) action is another form of flow control, which can be
+          used to repeat multiple instructions or make them obey a single
+          conditional. The instruction must include a C(block) field, containing
+          the list of instructions which are part of the block. In addition, it
+          may have a C(rescue) field, containing a list of instructions which
+          will be executed on error, and C(always), which may contain a list
+          of instructions to execute in all cases. If the C(locals) field is
+          defined, it must contain a table of local variables to define. Any
+          local variable defined by the instructions under C(block), C(rescue)
+          or C(always) will go out of scope once the block finishes executing.
         - C(create_group) creates a group. The name of the group must be
           provided using the C(group) field, which must be a valid name or a
           Jinja template that evaluates to a valid name.
@@ -157,8 +167,10 @@ class RcInstruction:
         if self._loop is None:
             return self.run_once(host_name, merged_vars, host_vars, script_vars)
         loop_values = self.evaluate_loop(host_name, merged_vars)
+        script_vars = script_vars.copy()
         for value in loop_values:
             merged_vars[self._loop_var] = value
+            script_vars[self._loop_var] = value
             if not self.run_once(host_name, merged_vars, host_vars, script_vars):
                 return False
         return True
@@ -370,9 +382,104 @@ class RciFail(RcInstruction):
         raise AnsibleRuntimeError(message)
 
 
+class RciBlock(RcInstruction):
+    def __init__(self, inventory, templar):
+        super().__init__(
+            inventory, templar, "block", ("block", "rescue", "always", "locals")
+        )
+        self._block = None
+        self._rescue = None
+        self._always = None
+        self._locals = None
+
+    def parse_action(self, record):
+        assert (
+            self._block is None
+            and self._rescue is None
+            and self._always is None
+            and self._locals is None
+        )
+        if "block" not in record:
+            raise AnsibleParserError("%s: missing 'block' field" % (self._action,))
+        self._block = self.parse_block(record, "block")
+        if "rescue" in record:
+            self._rescue = self.parse_block(record, "rescue")
+        else:
+            self._rescue = []
+        if "always" in record:
+            self._always = self.parse_block(record, "always")
+        else:
+            self._always = []
+        if "locals" in record:
+            if not isinstance(record["locals"], dict):
+                raise AnsibleParserError(
+                    "%s: 'locals' should be a dictionnary" % (self._action,)
+                )
+            for k, v in record["locals"].items():
+                if not isinstance(k, string_types):
+                    raise AnsibleParserError(
+                        "%s: locals identifiers must be strings" % (self._action,)
+                    )
+                if not isidentifier(k):
+                    raise AnsibleParserError(
+                        "%s: '%s' is not a valid identifier" % (self._action, k)
+                    )
+            self._locals = record["locals"]
+        else:
+            self._locals = {}
+
+    def parse_block(self, record, key):
+        if not isinstance(record[key], list):
+            raise AnsibleParserError(
+                "%s: '%s' field must contain a list of instructions"
+                % (self._action, key)
+            )
+        instructions = []
+        for record in record[key]:
+            instructions.append(
+                parse_instruction(self._inventory, self._templar, record)
+            )
+        return instructions
+
+    def execute_action(self, host_name, merged_vars, host_vars, script_vars):
+        assert not (
+            self._block is None
+            or self._rescue is None
+            or self._always is None
+            or self._locals is None
+        )
+        merged_vars = merged_vars.copy()
+        script_vars = script_vars.copy()
+        self._templar.available_variables = merged_vars
+        for key, value in self._locals.items():
+            result = self._templar.template(value)
+            script_vars[key] = result
+            merged_vars[key] = result
+        try:
+            try:
+                return self.run_block(
+                    self._block, host_name, merged_vars, host_vars, script_vars
+                )
+            except AnsibleError as e:
+                script_vars["reconstructed_error"] = str(e)
+                merged_vars["reconstructed_error"] = str(e)
+                return self.run_block(
+                    self._rescue, host_name, merged_vars, host_vars, script_vars
+                )
+        finally:
+            self.run_block(self._always, host_name, merged_vars, host_vars, script_vars)
+
+    def run_block(self, block, host_name, merged_vars, host_vars, script_vars):
+        for instruction in block:
+            if not instruction.run_for(host_name, host_vars, script_vars):
+                return False
+        return True
+
+
 INSTRUCTIONS = {
     "add_child": RciAddChild,
     "add_host": RciAddHost,
+    "block": RciBlock,
     "create_group": RciCreateGroup,
     "fail": RciFail,
     "set_fact": lambda i, t: RciSetVarOrFact(i, t, True),
@@ -381,6 +488,15 @@ INSTRUCTIONS = {
 }
 
 
+def parse_instruction(inventory, templar, record):
+    action = record["action"]
+    if action not in INSTRUCTIONS:
+        raise AnsibleParserError("Unknown action '%s'" % (action,))
+    instruction = INSTRUCTIONS[action](inventory, templar)
+    instruction.parse(record)
+    return instruction
+
+
 class InventoryModule(BaseInventoryPlugin):
     """Constructs groups based on lists of instructions."""
 
@@ -395,7 +511,7 @@ class InventoryModule(BaseInventoryPlugin):
         instr_src = self.get_option("instructions")
         instructions = []
         for record in instr_src:
-            instructions.append(self.get_instruction(record))
+            instructions.append(parse_instruction(self.inventory, self.templar, record))
         for host in inventory.hosts:
             try:
                 self.exec_for_host(host, instructions)
@@ -412,11 +528,3 @@ class InventoryModule(BaseInventoryPlugin):
         for instruction in instructions:
             if not instruction.run_for(host, host_vars, script_vars):
                 return
-
-    def get_instruction(self, record):
-        action = record["action"]
-        if action not in INSTRUCTIONS:
-            raise AnsibleParserError("Unknown action '%s'" % (action,))
-        instruction = INSTRUCTIONS[action](self.inventory, self.templar)
-        instruction.parse(record)
-        return instruction