feat(runtime): download tectonic on-the-fly into runs/_bin when not found; use for both poster and debug compiles

app.py

@@ -354,6 +354,54 @@ def render_overleaf_button(overleaf_b64):
     """
     return html
 
+def _get_tectonic_bin(logs):
+    """Return a usable path to the tectonic binary. Try PATH/common paths; if not found, download to runs/_bin."""
+    import shutil as _sh, tarfile, urllib.request, os
+    # 1) existing in PATH or common dirs
+    cands = [
+        "tectonic",
+        str(Path("/usr/local/bin/tectonic")),
+        str(Path("/usr/bin/tectonic")),
+        os.path.expanduser("~/.local/bin/tectonic"),
+        str((RUNS_DIR / "_bin" / "tectonic").resolve()),
+    ]
+    for c in cands:
+        if _sh.which(c) or Path(c).exists():
+            return c
+    # 2) download to runs/_bin
+    try:
+        url = "https://github.com/tectonic-typesetting/tectonic/releases/download/tectonic%400.15.0/tectonic-0.15.0-x86_64-unknown-linux-gnu.tar.gz"
+        bin_dir = RUNS_DIR / "_bin"
+        bin_dir.mkdir(parents=True, exist_ok=True)
+        tgz = bin_dir / "tectonic.tar.gz"
+        logs.append("⬇️ Downloading tectonic binary …")
+        with urllib.request.urlopen(url, timeout=60) as resp, open(tgz, "wb") as out:
+            out.write(resp.read())
+        with tarfile.open(tgz, "r:gz") as tf:
+            tf.extractall(path=bin_dir)
+        # find binary
+        tbin = None
+        for p in bin_dir.rglob("tectonic"):
+            if p.is_file() and os.access(p, os.X_OK):
+                tbin = p
+                break
+        if not tbin:
+            # make executable if needed
+            for p in bin_dir.rglob("tectonic"):
+                try:
+                    p.chmod(0o755)
+                    tbin = p
+                    break
+                except Exception:
+                    continue
+        if tbin:
+            logs.append(f"✅ Tectonic ready at {tbin}")
+            return str(tbin)
+        logs.append("⚠️ Tectonic binary not found after download.")
+    except Exception as e:
+        logs.append(f"⚠️ Tectonic download failed: {e}")
+    return None
+
 def _compile_poster_pdf(OUTPUT_DIR: Path, logs):
     """
     Compile output/poster_latex_proj/poster_output.tex into a PDF using an
@@ -380,20 +428,8 @@ def _compile_poster_pdf(OUTPUT_DIR: Path, logs):
 
         def _has(bin_name):
             return _sh.which(bin_name) is not None
-        def _candidate_tectonic():
-            import os
-            cands = [
-                "tectonic",
-                str(Path("/usr/local/bin/tectonic")),
-                str(Path("/usr/bin/tectonic")),
-                os.path.expanduser("~/.local/bin/tectonic"),
-            ]
-            for c in cands:
-                if _sh.which(c) or Path(c).exists():
-                    return c
-            return None
 
-        tbin = _candidate_tectonic()
+        tbin = _get_tectonic_bin(logs)
         if tbin:
             cmd = [tbin, tex_path.name]
             logs.append("▶ Compiling with tectonic …")
@@ -457,19 +493,7 @@ def _compile_tex_to_pdf(tex_path: Path, logs):
         import shutil as _sh, subprocess as _sp
         def _has(bin_name):
             return _sh.which(bin_name) is not None
-        def _candidate_tectonic():
-            import os
-            cands = [
-                "tectonic",
-                str(Path("/usr/local/bin/tectonic")),
-                str(Path("/usr/bin/tectonic")),
-                os.path.expanduser("~/.local/bin/tectonic"),
-            ]
-            for c in cands:
-                if _sh.which(c) or Path(c).exists():
-                    return c
-            return None
-        tbin = _candidate_tectonic()
+        tbin = _get_tectonic_bin(logs)
         if tbin:
             cmd = [tbin, tex_path.name]
             logs.append("▶ Compiling with tectonic …")