Why _default_context Exists (And Why It's Problematic)¶

# v0.5.0: Create minimal _default_context for backward compatibility only
# Operations should pass subject parameter instead of relying on this
self._default_context = OperationContext(
    user="system",
    groups=[],
    is_admin=is_admin,
    is_system=True,  # ← The problem
)

# OLD API (v0.4.x and earlier)
nx = NexusFS(
    backend,
    tenant_id="org_acme",  # ← Configured at init
    agent_id="bot1",       # ← Configured at init
    user_id="alice"        # ← Configured at init
)

# Operations used instance-level identity
nx.write("/file.txt", content)  # Implicitly uses nx.tenant_id, nx.user_id
nx.read("/file.txt")             # Implicitly uses nx.tenant_id, nx.user_id

# NEW API (v0.5.0+)
nx = NexusFS(backend)  # ← No user identity at init

# Operations require explicit context
ctx_alice = OperationContext(user="alice", groups=[], is_admin=False)
ctx_bob = OperationContext(user="bob", groups=[], is_admin=False)

nx.write("/file.txt", content, context=ctx_alice)  # Alice's operation
nx.read("/file.txt", context=ctx_bob)              # Bob's operation

# To support code that doesn't pass context:
def write(self, path, content, context=None):
    ctx = context or self._default_context  # ← Fallback for old code
    ...

# Note:
#     v0.5.0 BREAKING CHANGE: Removed tenant_id, user_id, and agent_id parameters.
#     Use the subject parameter in individual operations instead:
#
#         # Old way (v0.4.x - NO LONGER WORKS):
#         nx = NexusFS(backend, tenant_id="acme", agent_id="bot")
#         nx.read("/file.txt")
#
#         # New way (v0.5.0+):
#         nx = NexusFS(backend)
#         nx.read("/file.txt", subject=("agent", "bot"))

# Backward compatibility for code without context
self._default_context = OperationContext(
    user="system",
    is_system=True,  # ← Allow old code to work
)

# NEW code that forgets to pass context ALSO gets system bypass!
nx = NexusFS(backend, enforce_permissions=True)
nx.write("/admin/secrets.txt", data)  # ⚠️ Forgot context parameter!
#                                     # Gets system bypass anyway!

def write(self, path, content, context=None):
    # Enforce context when permissions are enabled
    if self._enforce_permissions and context is None:
        raise ValueError(
            "context parameter is required when enforce_permissions=True.\n"
            "For backward compatibility, set enforce_permissions=False."
        )

    ctx = context or self._default_context  # Only used if permissions disabled
    ...

# Backward compatibility but SECURE by default
self._default_context = OperationContext(
    user="anonymous",      # Not "system"
    groups=[],
    is_admin=False,        # Not True
    is_system=False,       # ← CRITICAL: No bypass
)

def write(self, path, content, context=None):
    if context is None:
        warnings.warn(
            "Calling write() without context is deprecated and will be removed in v0.7.0. "
            "Pass context=OperationContext(...) explicitly.",
            DeprecationWarning,
            stacklevel=2
        )

    ctx = context or self._default_context
    ...

# Personal filesystem browser
nx = NexusFS(backend, enforce_permissions=False)
nx.write("/my-notes.txt", data)  # Works fine, no context needed

# Production deployment
nx = NexusFS(backend, enforce_permissions=True)

# RPC server handles request from user Alice
def handle_write_request(path, content):
    # ⚠️ Doesn't pass context - uses _default_context!
    nx.write(path, content)  # System bypass - Alice can access EVERYTHING

# RPC server handles request from user Bob
def handle_read_request(path):
    # ⚠️ Doesn't pass context - uses _default_context!
    return nx.read(path)  # System bypass - Bob can read EVERYTHING

# If we change to is_system=False:
self._default_context = OperationContext(
    user="system",
    is_system=False,  # ← No longer bypasses
)

# Old code that relied on no permission checks will break:
nx = NexusFS(backend)  # Old code, no enforce_permissions param
nx.write("/file.txt", data)  # ❌ May fail with permission errors

# src/nexus/core/nexus_fs.py:202-207
self._default_context = OperationContext(
    user="anonymous",      # ← Change from "system"
    groups=[],
    is_admin=False,        # ← Never True
    is_system=False,       # ← CRITICAL FIX
)

# Add validation
def write(self, path, content, context=None):
    if self._enforce_permissions and context is None:
        raise ValueError(
            "context is required when enforce_permissions=True. "
            "Pass context=OperationContext(user='...', groups=[], is_admin=False)"
        )
    ...

# For old code without permissions:
nx = NexusFS(backend, enforce_permissions=False)  # ← Explicitly disable
nx.write("/file.txt", data)  # Works without context

# For new code with permissions:
nx = NexusFS(backend, enforce_permissions=True)   # ← Enable security
ctx = OperationContext(user="alice", groups=[], is_admin=False)
nx.write("/file.txt", data, context=ctx)  # ← Must pass context