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fix: prevent duplicate channels from concurrent add/join requests

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Two near-simultaneous POSTs to /api/channels/join (observed 7 ms apart
in demo-server logs) each found a different free slot and both
succeeded, producing two entries for the same channel name on the
device. This also shifted the sidebar so each channel rendered the
next one's messages.

- Wrap free-slot detection + set_channel in a module-level lock so
  concurrent requests serialize instead of racing.
- Idempotency: if a channel with this name already exists, return the
  existing slot with already_existed=true instead of creating a
  duplicate. Applies to both POST /api/channels and /api/channels/join
  (skipped when caller targets an explicit index).
- Disable submit buttons on create/join forms while a request is in
  flight, and guard against double-registration of the channel-link
  click delegate to stop a single click from firing N POSTs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
MarekWo
2026-04-22 20:45:39 +02:00
parent 66c378c17d
commit cbcdbdcae9
3 changed files with 155 additions and 77 deletions
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202
app/routes/api.py
View File

@@ -9,6 +9,7 @@ import json
import re
import base64
import struct
import threading
import time
import requests
from Crypto.Cipher import AES
@@ -42,6 +43,11 @@ _channels_cache = None
_channels_cache_timestamp = 0
CHANNELS_CACHE_TTL = 30 # seconds
# Serializes channel create/join to avoid duplicates from concurrent requests.
# Without this, two near-simultaneous POSTs both find the same "first free"
# slot (or adjacent slots) and each succeeds, producing duplicate entries.
_channel_write_lock = threading.Lock()
# Cache for contacts/detailed to reduce USB calls (4 calls per request!)
# Contacts change infrequently, 60s cache is safe
_contacts_detailed_cache = None
@@ -1570,36 +1576,58 @@ def create_channel():
'error': 'Channel name can only contain letters, numbers, _ and -'
}), 400
success, message, key = cli.add_channel(name)
# Serialize create/join so concurrent requests can't both pick a free
# slot for the same name and produce duplicates.
with _channel_write_lock:
# Idempotency: if a channel with this name already exists, return it
# instead of creating a second slot. Compare case-insensitively.
invalidate_channels_cache() # force fresh read from device
success_ch, existing = get_channels_cached(force_refresh=True)
if success_ch:
for ch in existing:
if ch.get('name', '').lower() == name.lower():
logger.info(f"Channel '{name}' already exists at slot {ch['index']}, returning existing")
return jsonify({
'success': True,
'message': f"Channel '{name}' already exists at slot {ch['index']}",
'channel': {
'index': ch['index'],
'name': ch.get('name', name),
'key': ch.get('key', ''),
},
'already_existed': True,
}), 200
if success:
invalidate_channels_cache() # Clear cache to force refresh
success, message, key = cli.add_channel(name)
# Build response
response = {
'success': True,
'message': message,
'channel': {
'name': name,
'key': key
if success:
invalidate_channels_cache() # Clear cache to force refresh
# Build response
response = {
'success': True,
'message': message,
'channel': {
'name': name,
'key': key
}
}
}
# Check channel count for soft limit warning
success_ch, channels = get_channels_cached()
if success_ch and len(channels) > 7:
response['warning'] = (
f'You now have {len(channels)} channels. '
'Some devices may only support up to 8 channels. '
'Check your device specifications if you experience issues.'
)
# Check channel count for soft limit warning
success_ch, channels = get_channels_cached()
if success_ch and len(channels) > 7:
response['warning'] = (
f'You now have {len(channels)} channels. '
'Some devices may only support up to 8 channels. '
'Check your device specifications if you experience issues.'
)
return jsonify(response), 201
else:
return jsonify({
'success': False,
'error': message
}), 500
return jsonify(response), 201
else:
return jsonify({
'success': False,
'error': message
}), 500
except Exception as e:
logger.error(f"Error creating channel: {e}")
@@ -1641,65 +1669,89 @@ def join_channel():
'error': 'Key is required for channels not starting with #'
}), 400
# Auto-detect free slot if not provided
if 'index' in data:
index = int(data['index'])
else:
# Find first free slot (1-40, skip 0 which is Public)
# Hard limit: 40 channels (most LoRa devices support up to 40)
# Soft limit: 7 channels (some devices may have lower limits)
success_ch, channels = get_channels_cached()
# Serialize create/join so concurrent requests can't both pick a free
# slot for the same name and produce duplicates.
with _channel_write_lock:
# Fresh read from device before picking slot or checking duplicates.
invalidate_channels_cache()
success_ch, channels = get_channels_cached(force_refresh=True)
if not success_ch:
return jsonify({
'success': False,
'error': 'Failed to get current channels'
}), 500
used_indices = {ch['index'] for ch in channels}
index = None
for i in range(1, 41): # Max 40 channels (hard limit)
if i not in used_indices:
index = i
break
# Idempotency: if a channel with this name already exists, return
# the existing slot instead of creating a duplicate. Skip this check
# only when caller explicitly targets an index (explicit overwrite).
if 'index' not in data:
for ch in channels:
if ch.get('name', '').lower() == name.lower():
logger.info(f"Channel '{name}' already exists at slot {ch['index']}, returning existing")
return jsonify({
'success': True,
'message': f"Already joined channel \"{name}\" at slot {ch['index']}",
'channel': {
'index': ch['index'],
'name': ch.get('name', name),
'key': ch.get('key', '') or 'auto-generated',
},
'already_existed': True,
}), 200
if index is None:
# Auto-detect free slot if not provided
if 'index' in data:
index = int(data['index'])
used_indices = {ch['index'] for ch in channels}
else:
# Find first free slot (1-40, skip 0 which is Public)
# Hard limit: 40 channels (most LoRa devices support up to 40)
# Soft limit: 7 channels (some devices may have lower limits)
used_indices = {ch['index'] for ch in channels}
index = None
for i in range(1, 41): # Max 40 channels (hard limit)
if i not in used_indices:
index = i
break
if index is None:
return jsonify({
'success': False,
'error': 'No free channel slots available (max 40 channels)'
}), 400
success, message = cli.set_channel(index, name, key)
if success:
invalidate_channels_cache() # Clear cache to force refresh
# Build response
response = {
'success': True,
'message': f'Joined channel "{name}" at slot {index}',
'channel': {
'index': index,
'name': name,
'key': key if key else 'auto-generated'
}
}
# Add warning if exceeding soft limit (7 channels)
# Some older/smaller devices may only support 8 channels total
channel_count = len(used_indices) + 1 # +1 for newly added channel
if channel_count > 7:
response['warning'] = (
f'You now have {channel_count} channels. '
'Some devices may only support up to 8 channels. '
'Check your device specifications if you experience issues.'
)
return jsonify(response), 200
else:
return jsonify({
'success': False,
'error': 'No free channel slots available (max 40 channels)'
}), 400
success, message = cli.set_channel(index, name, key)
if success:
invalidate_channels_cache() # Clear cache to force refresh
# Build response
response = {
'success': True,
'message': f'Joined channel "{name}" at slot {index}',
'channel': {
'index': index,
'name': name,
'key': key if key else 'auto-generated'
}
}
# Add warning if exceeding soft limit (7 channels)
# Some older/smaller devices may only support 8 channels total
channel_count = len(used_indices) + 1 # +1 for newly added channel
if channel_count > 7:
response['warning'] = (
f'You now have {channel_count} channels. '
'Some devices may only support up to 8 channels. '
'Check your device specifications if you experience issues.'
)
return jsonify(response), 200
else:
return jsonify({
'success': False,
'error': message
}), 500
'error': message
}), 500
except Exception as e:
logger.error(f"Error joining channel: {e}")

22
app/static/js/app.js
View File

@@ -761,8 +761,12 @@ function setupEventListeners() {
document.getElementById('createChannelForm').addEventListener('submit', async function(e) {
e.preventDefault();
const submitBtn = this.querySelector('button[type="submit"]');
if (submitBtn && submitBtn.disabled) return; // in-flight guard
const name = document.getElementById('newChannelName').value.trim();
if (submitBtn) submitBtn.disabled = true;
try {
const response = await fetch('/api/channels', {
method: 'POST',
@@ -775,7 +779,10 @@ function setupEventListeners() {
const data = await response.json();
if (data.success) {
showNotification(`Channel "${name}" created!`, 'success');
const msg = data.already_existed
? `Channel "${name}" already exists.`
: `Channel "${name}" created!`;
showNotification(msg, data.already_existed ? 'info' : 'success');
// Show warning if returned (e.g., exceeding soft limit of 7 channels)
if (data.warning) {
@@ -795,6 +802,8 @@ function setupEventListeners() {
}
} catch (error) {
showNotification('Failed to create channel', 'danger');
} finally {
if (submitBtn) submitBtn.disabled = false;
}
});
@@ -802,6 +811,9 @@ function setupEventListeners() {
document.getElementById('joinChannelFormSubmit').addEventListener('submit', async function(e) {
e.preventDefault();
const submitBtn = this.querySelector('button[type="submit"]');
if (submitBtn && submitBtn.disabled) return; // in-flight guard
const name = document.getElementById('joinChannelName').value.trim();
const key = document.getElementById('joinChannelKey').value.trim().toLowerCase();
@@ -817,6 +829,7 @@ function setupEventListeners() {
return;
}
if (submitBtn) submitBtn.disabled = true;
try {
const payload = { name: name };
if (key) {
@@ -834,7 +847,10 @@ function setupEventListeners() {
const data = await response.json();
if (data.success) {
showNotification(`Joined channel "${name}"!`, 'success');
const msg = data.already_existed
? `Already joined channel "${name}".`
: `Joined channel "${name}"!`;
showNotification(msg, data.already_existed ? 'info' : 'success');
// Show warning if returned (e.g., exceeding soft limit of 7 channels)
if (data.warning) {
@@ -855,6 +871,8 @@ function setupEventListeners() {
}
} catch (error) {
showNotification('Failed to join channel', 'danger');
} finally {
if (submitBtn) submitBtn.disabled = false;
}
});

8
app/static/js/message-utils.js
View File

@@ -360,10 +360,18 @@ async function joinAndSwitchToChannel(channelName) {
/**
* Initialize channel link click handlers using event delegation
*/
let _channelLinkHandlersInitialized = false;
function initializeChannelLinkHandlers() {
// Guard against double registration - otherwise one click fires N handlers
// and sends N duplicate POSTs to /api/channels/join.
if (_channelLinkHandlersInitialized) return;
_channelLinkHandlersInitialized = true;
document.addEventListener('click', function(e) {
if (e.target.classList.contains('channel-link')) {
e.preventDefault();
// Swallow clicks while this link is already handling a request.
if (e.target.classList.contains('loading')) return;
const channelName = e.target.getAttribute('data-channel-name');
if (channelName) {