viewer: proven read-only; its database check no longer creates SQLite files

tests/read_only.rs: with every file and folder of the books read-only on
disk, the viewer loads, follows live, and renders every page (receipts
included), and every file keeps its bytes, size, and mtime; on writable
books nothing appears beside the database either.

Found: with no writer running, the snapshot's read-only open of the WAL
database had to create -wal/-shm (a write into the books) and failed on
read-only books, showing a false "database unreadable". With no -wal
file no writer has it open, so check_disk now reads it immutable.

just mutants-gui sweeps only what the viewer must get right: the view
model's numbers (books.rs) and their formatting (fmt.rs); 179 mutants.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

crates/ledger-gui/tests/read_only.rs

@@ -0,0 +1,177 @@
+//! The viewer is read-only: it never writes into the books.
+//!
+//! ```text
+//! GOAL: Stephen, Kat, and the CPA can open the books in the viewer
+//!       without any chance of changing them.
+//! SOURCE: DECISION (AGENTS.md: "read-only and live ... It never writes;
+//!         changes go through you").
+//! INVARIANTS:
+//!   1. With every file and folder of the books made read-only on disk,
+//!      the viewer still loads, follows live, and renders every page
+//!      (receipts included), and reports the books whole.
+//!   2. Afterwards every file holds the same bytes, size, and
+//!      modification time, and no file has appeared or gone; on writable
+//!      books too (no SQLite -wal or -shm created beside the database).
+//! ```
+
+use egui_kittest::Harness;
+use ledger_gui::data::{self, Loader, Update};
+use ledger_gui::nav::{ReportKind, Route};
+use ledger_gui::{App, Source};
+use std::collections::BTreeMap;
+use std::os::unix::fs::PermissionsExt;
+use std::path::{Path, PathBuf};
+use std::time::{Duration, SystemTime};
+
+type Tree = BTreeMap<PathBuf, (Vec<u8>, u64, SystemTime)>;
+
+fn tree(root: &Path) -> Tree {
+    fn walk(root: &Path, d: &Path, out: &mut Tree) {
+        for e in std::fs::read_dir(d).unwrap() {
+            let p = e.unwrap().path();
+            let m = std::fs::symlink_metadata(&p).unwrap();
+            if m.is_dir() {
+                walk(root, &p, out);
+            } else {
+                out.insert(
+                    p.strip_prefix(root).unwrap().to_path_buf(),
+                    (std::fs::read(&p).unwrap(), m.len(), m.modified().unwrap()),
+                );
+            }
+        }
+    }
+    let mut out = BTreeMap::new();
+    walk(root, root, &mut out);
+    out
+}
+
+/// Every file 0444 and folder 0555 (`read_only`), or back to 0644/0755.
+fn set_read_only(root: &Path, read_only: bool) {
+    fn walk(p: &Path, ro: bool) {
+        let m = std::fs::symlink_metadata(p).unwrap();
+        if m.is_dir() {
+            if !ro {
+                std::fs::set_permissions(p, std::fs::Permissions::from_mode(0o755)).unwrap();
+            }
+            for e in std::fs::read_dir(p).unwrap() {
+                walk(&e.unwrap().path(), ro);
+            }
+            if ro {
+                std::fs::set_permissions(p, std::fs::Permissions::from_mode(0o555)).unwrap();
+            }
+        } else {
+            let mode = if ro { 0o444 } else { 0o644 };
+            std::fs::set_permissions(p, std::fs::Permissions::from_mode(mode)).unwrap();
+        }
+    }
+    walk(root, read_only);
+}
+
+#[test]
+fn the_viewer_never_writes_into_the_books() {
+    let dir = std::env::temp_dir().join(format!("ledger-gui-ro-{}", std::process::id()));
+    if dir.exists() {
+        set_read_only(&dir, false);
+        std::fs::remove_dir_all(&dir).unwrap();
+    }
+    let today = chrono::NaiveDate::from_ymd_opt(2026, 10, 12).unwrap();
+    ledger_gui::demo::build(&dir, today).unwrap();
+    let source = Source {
+        data_dir: dir.join("books-data"),
+        books_toml: Some(dir.join("wanderbooks.toml")),
+        today: Some(today),
+    };
+    set_read_only(&dir, true);
+    let before = tree(&dir);
+
+    // Load once; the books are whole (the database matches the journal).
+    let books = data::load_once(&source).expect("loads from read-only books");
+    assert!(
+        matches!(books.integrity.disk, ledger_store::DiskCheck::Matches),
+        "read-only books reported as {:?}",
+        books.integrity.disk
+    );
+    assert!(books.integrity.file_failures.is_empty());
+    let entry = books.entries.last().map(|e| e.id).unwrap();
+    let cash = books.cash_accounts[0];
+    let doc = books.attachments.last().map(|a| a.id).unwrap();
+    let event = books.seq();
+    drop(books);
+
+    // Follow live for a moment.
+    let loader = Loader::spawn(source.clone(), egui::Context::default());
+    let deadline = std::time::Instant::now() + Duration::from_secs(20);
+    let mut live = false;
+    while std::time::Instant::now() < deadline {
+        match loader.wait(Duration::from_millis(200)) {
+            Some(Update::Books(_)) => {
+                live = true;
+                break;
+            }
+            Some(Update::Error(e)) => panic!("live loader on read-only books: {e}"),
+            _ => {}
+        }
+    }
+    assert!(live, "the live loader never delivered the books");
+
+    // Render every page.
+    let routes = [
+        Route::Overview,
+        Route::Transactions,
+        Route::Entry(entry),
+        Route::Cash,
+        Route::Profit,
+        Route::Reports(ReportKind::ProfitAndLoss),
+        Route::Reports(ReportKind::BalanceSheet),
+        Route::Reports(ReportKind::CashFlow),
+        Route::Reports(ReportKind::TrialBalance),
+        Route::Reports(ReportKind::GeneralLedger),
+        Route::Reports(ReportKind::TaxSummary),
+        Route::Reports(ReportKind::Vendor1099),
+        Route::Reports(ReportKind::ArAging),
+        Route::Accounts,
+        Route::Account(cash),
+        Route::Document(doc),
+        Route::JournalEvent(event),
+        Route::Documents,
+        Route::Company,
+        Route::Journal,
+    ];
+    for route in routes {
+        let books = data::load_once(&source).unwrap();
+        let mut harness = Harness::builder()
+            .with_size(egui::vec2(1280.0, 800.0))
+            .build_eframe(|cc| App::with_books(&cc.egui_ctx, books, false, route.clone()));
+        harness.run_steps(3);
+        let ctx = harness.ctx.clone();
+        harness.state_mut().settle(&ctx);
+        harness.run_steps(2);
+    }
+
+    let after = tree(&dir);
+    set_read_only(&dir, false);
+    let changed: Vec<_> = before
+        .keys()
+        .chain(after.keys())
+        .filter(|k| before.get(*k) != after.get(*k))
+        .collect();
+    assert!(changed.is_empty(), "the viewer changed {changed:?}");
+
+    // On writable books too: loading and checking creates nothing (no
+    // SQLite -wal or -shm beside the database).
+    let before = tree(&dir);
+    let books = data::load_once(&source).unwrap();
+    assert!(matches!(
+        books.integrity.disk,
+        ledger_store::DiskCheck::Matches
+    ));
+    drop(books);
+    let after = tree(&dir);
+    assert_eq!(
+        before.keys().collect::<Vec<_>>(),
+        after.keys().collect::<Vec<_>>(),
+        "the viewer created or removed files in writable books"
+    );
+    assert!(before == after, "the viewer changed writable books");
+    let _ = std::fs::remove_dir_all(&dir);
+}

crates/ledger-store/src/snapshot.rs

@@ -229,10 +229,28 @@ impl Snapshot {
         if !path.exists() {
             return DiskCheck::Missing;
         }
-        let mut conn = match Connection::open_with_flags(
+        // A WAL reader needs the -wal and -shm files, and creates them if
+        // they're missing: a write into the books, and a failure when the
+        // books are read-only to this user. With no -wal file no writer
+        // has the database open (SQLite deletes it when the last one
+        // closes), so it can't change under us: read it as an immutable
+        // file, creating nothing (found by the viewer's read-only test,
+        // 2026-10-14).
+        let live = path.with_extension("db-wal").exists();
+        let opened = if live {
+            Connection::open_with_flags(
                 &path,
                 OpenFlags::SQLITE_OPEN_READ_ONLY | OpenFlags::SQLITE_OPEN_NO_MUTEX,
-        ) {
+            )
+        } else {
+            Connection::open_with_flags(
+                immutable_uri(&path),
+                OpenFlags::SQLITE_OPEN_READ_ONLY
+                    | OpenFlags::SQLITE_OPEN_NO_MUTEX
+                    | OpenFlags::SQLITE_OPEN_URI,
+            )
+        };
+        let mut conn = match opened {
             Ok(c) => c,
             Err(e) => return DiskCheck::Unreadable(e.to_string()),
         };
@@ -286,3 +304,18 @@ impl Snapshot {
         read_attachment_bytes(&self.conn, &self.blobs, attachment_id)
     }
 }
+
+/// `file:` URI for `path`, read-only and immutable (sqlite.org/uri.html):
+/// every byte outside the unreserved set is percent-encoded.
+fn immutable_uri(path: &Path) -> String {
+    let mut out = String::from("file:");
+    for b in path.to_string_lossy().bytes() {
+        if b.is_ascii_alphanumeric() || b"/-_.~".contains(&b) {
+            out.push(b as char);
+        } else {
+            out.push_str(&format!("%{b:02X}"));
+        }
+    }
+    out.push_str("?mode=ro&immutable=1");
+    out
+}

justfile

@@ -94,11 +94,13 @@ mutants-mcp:
     mkdir -p {{mutants_tmp}}
     MCP_CASES=16 PROPTEST_MAX_SHRINK_ITERS=0 TMPDIR={{mutants_tmp}} cargo mutants -p ledger-mcp --test-package ledger-mcp -j 4 --timeout 600 -o mutants-mcp
 
-# Mutation testing of the viewer's logic (formatting, file names, history,
-# search, ticks, the view model, the attention list; not the painting).
+# Mutation testing of the viewer where it matters: the numbers it shows
+# (the view model) and how it writes them (formatting). Its read-only
+# promise has its own test (tests/read_only.rs); search, history, ticks,
+# and the attention list are spec-tested, not swept.
 mutants-gui:
     mkdir -p {{mutants_tmp}}
-    PROPTEST_MAX_SHRINK_ITERS=0 TMPDIR={{mutants_tmp}} cargo mutants -p ledger-gui --test-package ledger-gui -j 4 --timeout 60 -o mutants-gui -f crates/ledger-gui/src/fmt.rs -f crates/ledger-gui/src/export.rs -f crates/ledger-gui/src/nav.rs -f crates/ledger-gui/src/attention.rs -f crates/ledger-gui/src/books.rs -f crates/ledger-gui/src/palette.rs -f crates/ledger-gui/src/charts.rs
+    PROPTEST_MAX_SHRINK_ITERS=0 TMPDIR={{mutants_tmp}} cargo mutants -p ledger-gui --test-package ledger-gui -j 4 --timeout 180 -o mutants-gui -f crates/ledger-gui/src/books.rs -f crates/ledger-gui/src/fmt.rs
 
 # Benchmarks of the core's hot paths (no budget gates them yet)
 bench: