Engineering Notes

When you build a desktop app for distribution, macOS may greet users with “cannot be opened because the developer cannot be verified,” and Windows SmartScreen may show “Windows protected your PC” for an unrecognized publisher. Signing and notarizing an app is how you avoid these warnings, but what exactly does that signature prove — and what does it not prove? The distinction is easy to get wrong. Here’s a breakdown of two systems that look similar on the surface but play different roles: Apple’s Notarization and Windows’ Authenticode. Note: Code signing is the umbrella term for attaching a cryptographic signature to an executable so that its author’s identity and the …

Start a Flask app and the terminal prints a familiar line: “WARNING: This is a development server. Do not use it in a production deployment.” Yet plenty of desktop apps bundle that same local Flask server as their actual runtime and keep it running on the user’s machine for the life of the session. That looks like ignoring the warning outright, but the underlying assumptions have actually changed. This article works through what has to change for that warning to become safe to set aside — and what you still have to handle yourself, or it turns into a real bug. Note: WSGI (Web Server Gateway Interface) is the standard …

What happens if the power cuts out while a process is writing to a config file? Or if antivirus software on Windows briefly locks a file mid-write? If you naively overwrite a file with open(path, ‘w’), whatever partial content existed at the moment of interruption is what remains on disk. For JSON, that usually means broken syntax — json.load() throws on the next startup, and the entire configuration is effectively lost. This article walks through a standard technique for preventing that: writing to a temporary file first, then swapping it in atomically. Note: “Atomic” here means an operation either completes entirely or doesn’t happen at all — there’s no partial, …

What happens when a GUI app and a separate background process both try to write to the same configuration file at the same time? If the timing is bad, one write clobbers the other, and in the worst case the file ends up corrupted. File locking is the standard answer to this “concurrent writes from multiple processes” problem. Trying to implement it in pure Python across both Unix-like systems and Windows runs straight into a wall: the two platforms expose completely different APIs. This article walks through that difference from the ground up. Note: “lock” here means inter-process locking — coordinating multiple separate processes on the same machine. That’s different …

Placing corrections exactly where an LLM is tempted to sound plausible instead of right Background The support chatbot on our landing page feeds the contents of a knowledge base to an LLM as its system prompt, then lets it answer whatever a user asks. Running it in practice surfaced something a generic instruction like “answer accurately” doesn’t cover: the same plausible-sounding mistake, repeated consistently, for certain specific kinds of questions. An LLM has absorbed a huge amount of general technical knowledge. That’s exactly why, the moment it’s asked something, it tends to combine that general knowledge into an answer that sounds thoroughly convincing. The tricky part is that the answer …

A support chatbot needs to be designed for what it won’t answer, not just what it will Background The support chatbot on our landing page has a simple job: answer questions about the product’s features, pricing, and troubleshooting. But a conversational AI built to answer questions shares a common weakness. Left unguarded, it tends to comply with requests designed to surface its own system prompt verbatim, or with phrasing that falsely claims special authority to unlock a different kind of answer. The chatbot works by feeding a knowledge base file to the LLM as its system prompt, then letting it answer whatever the user asks. Rather than implementing this defense …

The free plan wasn’t actually “no registration required” — fixing a landing page claim that contradicted itself Background An external marketing review flagged what looked like a contradiction in our landing page’s FAQ. Checking it against the actual code confirmed the claim was accurate. Two statements coexisted on the same page: Quick Start, step 2: “Register with your email to start for free” The FAQ answer: “The Free plan requires no registration and allows up to 1 site, 3 runs per month — completely free.” One says “please register.” The other says “no registration needed.” Checking the implementation confirmed that the Free plan requires an email address to be registered …

A tabbed form that silently refused to submit — required fields hidden behind another tab Background The site edit modal kept accumulating fields — site name, category, SSH connection details, WordPress install location — until editing anything meant scrolling up and down a single long form to find the right field. To clean this up, we split it into three tabs: “Registration info,” “SSH,” and “WordPress info.” That change broke form submission itself, in a way that was hard to spot at first. What tabbing broke The tab implementation itself is straightforward. Each tab’s fields live in a <div class=”site-tab-content” data-tab=”…”>, and CSS toggles which one is visible. .site-tab-content { …

SPF, DKIM, and DMARC together — why the missing DMARC record was blocking registration emails Background Registration confirmation emails were not reliably reaching users on Gmail and Outlook outside Japan — sometimes landing in spam, sometimes not arriving at all. Investigation pointed to a single root cause: the wpmm.jp domain had SPF and DKIM configured, but no DMARC record. What each of the three does SPF (Sender Policy Framework) declares in DNS which IP addresses are authorized to send mail for a domain. Receiving servers check the sending IP against the SPF record to confirm the source is legitimate. DKIM (DomainKeys Identified Mail) adds a cryptographic signature to the message …

Resuming email verification after the app is closed — the pending_email state that prevents re-sending Background On first launch, the desktop app asks the user for an email address, sends a confirmation email, and completes registration once the user clicks the link in that email. There was a trap here for anyone who closed the app before clicking the link. Email does not always arrive immediately. Closing the app with the intention of clicking the link once the email shows up, then reopening later, is a perfectly natural way to use it. But on restart the app decided that registration was still incomplete and showed the first-launch screen (the email …