fix(api): return prompt error response instead of hanging when a SAS …

api/docs/diagrams/code-execution-overview.md

@@ -0,0 +1,38 @@
+# Code execution & session diagrams
+
+Mermaid diagrams describing how `@sasjs/server` executes submitted code
+(SAS/JS/PY/R) against pooled "sessions". Written for fast context-loading by
+an AI coding agent: each diagram is self-contained, node/edge labels carry
+the actual file:line references, and prose is kept to the minimum needed to
+disambiguate the diagram.
+
+| File | Covers |
+|---|---|
+| [session-lifecycle.md](session-lifecycle.md) | `SessionState` state machine; how it differs between the SAS runtime and the JS/PY/R runtimes |
+| [sas-execution-handshake.md](sas-execution-handshake.md) | The SYSIN/AUTOEXEC file-swap mechanism a SAS session uses to turn one long-lived `sas` process into a single-use execution slot |
+| [request-execution-flow.md](request-execution-flow.md) | End-to-end flowchart from HTTP request to response, covering session-pool acquisition and both runtime branches (SAS vs JS/PY/R) |
+
+## Core concept
+
+A "session" is not a request-scoped object. It is a pooled, reusable
+execution slot with a filesystem folder (`session.path`) and a state
+(`SessionState`). For the SAS runtime specifically, a session also owns one
+real OS process, spawned at session-*creation* time and consumed by exactly
+one code submission (see `sas-execution-handshake.md`). For JS/PY/R, a
+session is just an ID + folder; the actual interpreter process is spawned
+fresh per request inside `processProgram`.
+
+## Key source files
+
+- `api/src/controllers/internal/Session.ts` — session pool + lifecycle
+  (`SessionController`, `SASSessionController`), `SessionState` transitions.
+- `api/src/controllers/internal/processProgram.ts` — writes the submitted
+  code into the session and drives it to completion/failure, per runtime.
+- `api/src/controllers/internal/Execution.ts` — `ExecutionController`,
+  the entry point controllers call; acquires a session, calls
+  `processProgram`, reads back `log.log`/`webout.txt`/headers, builds the
+  HTTP response (or a `SessionExecutionError` on failure).
+- `api/src/controllers/internal/create{SAS,JS,Python,R}Program.ts` —
+  per-runtime code templating (wraps the user's submitted code with
+  boilerplate: variable injection, `_webout` redirection, etc).
+- `api/src/types/Session.ts` — `SessionState` enum and `Session` interface.

api/docs/diagrams/request-execution-flow.md

@@ -0,0 +1,64 @@
+# Request execution flow (end-to-end)
+
+Full path from an inbound HTTP request to the HTTP response, covering
+session-pool acquisition and the two structurally different runtime
+branches: SAS (reuses a parked long-lived process) vs JS/PY/R (spawns a
+fresh interpreter process per request).
+
+```mermaid
+flowchart TD
+    A["HTTP request<br/>POST /SASjsApi/code/execute<br/>or /SASjsApi/stp/execute"] --> B["Controller<br/>code.ts / stp.ts<br/>try/catch wrapper"]
+    B --> C["ExecutionController.executeProgram<br/>or .executeFile<br/>Execution.ts:58-91"]
+    C --> D{"session already provided?<br/>(e.g. file-upload flow)"}
+    D -- no --> E["getSessionController(runTime).getSession()<br/>Session.ts:59-69"]
+    D -- yes --> F["use provided session"]
+    E --> G{"pending session<br/>available in pool?"}
+    G -- yes --> H["reuse it"]
+    G -- no --> I["createSession()<br/>SAS: spawns a real sas process, see<br/>sas-execution-handshake.md<br/>JS/PY/R: cheap - folder + id only"]
+    H --> J["pre-warm: if pool has fewer<br/>than 3 pending, fire off more<br/>createSession() calls (not awaited)<br/>Session.ts:66"]
+    I --> J
+    F --> K
+    J --> K["session.state = running<br/>Execution.ts:96"]
+    K --> L["write webout.txt (empty),<br/>reqHeaders.txt<br/>Execution.ts:103-107"]
+    L --> M["processProgram(...)<br/>processProgram.ts:16-27"]
+
+    M --> N{"runTime?"}
+
+    N -- SAS --> O["createSASProgram():<br/>wrap user code with _webout<br/>filename, macro vars, autoexec"]
+    O --> P["write code.sas via .bkp + rename<br/>processProgram.ts:48-49"]
+    P --> Q["poll: while session.state !== completed<br/>processProgram.ts:52-58"]
+    Q -- "state becomes completed" --> R["resolve"]
+    Q -- "state becomes failed" --> S["throw Error(session.failureReason)"]
+
+    N -- "JS / PY / R" --> T["createJSProgram / createPythonProgram /<br/>createRProgram: wrap user code similarly"]
+    T --> U["write code file; spawn interpreter<br/>fresh via execFile; pipe stdout/stderr<br/>into a log.log write stream<br/>processProgram.ts:108-134"]
+    U -- "exit 0" --> R
+    U -- "exit non-zero" --> S
+
+    R --> V["read log.log, webout.txt,<br/>stpsrv_header.txt<br/>Execution.ts:128-131"]
+    V --> W["build httpHeaders + result<br/>return ExecuteReturnRaw<br/>Execution.ts:128-151"]
+    W --> X["Controller: res.send(result)<br/>HTTP 200"]
+
+    S --> Y["catch in Execution.ts:109-126:<br/>read whatever log exists,<br/>throw SessionExecutionError(message, log)"]
+    Y --> Z["Controller catch block<br/>rethrow { code: 400, status: 'failure',<br/>message, error, log }"]
+    Z --> AA["res.status(err.code).send(err)<br/>HTTP 400 with complete log"]
+```
+
+## Notes
+
+- **Session pool is per-runtime.** SAS sessions and JS/PY/R sessions live in
+  separate controllers/pools (`process.sasSessionController` vs
+  `process.sessionController`, `Session.ts:239-253`) - a pending SAS session
+  is never handed out for a JS request or vice versa.
+- **The SAS branch and the JS/PY/R branch converge** on the same
+  success/failure signal shape: `session.state` becomes `completed` or
+  `failed` either way, and both paths flow through the same log/webout
+  reading logic in `Execution.ts` afterward. The mechanics of *how* that
+  state gets set differ substantially (see `session-lifecycle.md`).
+- **`includePrintOutput`** (SAS only) additionally appends `output.lst`
+  content to the result when debug mode is on - omitted above for brevity;
+  see `Execution.ts:135-142`.
+- **`triggerProgram`/`triggerCode`** (fire-and-forget variants, not shown)
+  call the same `ExecutionController` methods without awaiting them and
+  immediately return `{ sessionId }`; the client polls
+  `GET /SASjsApi/session/{sessionId}/state` separately.

api/docs/diagrams/sas-execution-handshake.md

@@ -0,0 +1,81 @@
+# SAS session execution handshake
+
+How one long-lived `sas` process is turned into a single-use execution slot
+via a SYSIN file-swap trick. This is the mechanism specific to
+`RunTimeType.SAS`; JS/PY/R sessions spawn a fresh interpreter process per
+request instead (see `request-execution-flow.md`).
+
+```mermaid
+sequenceDiagram
+    participant Client as HTTP client
+    participant Ctrl as Controller<br/>(code.ts / stp.ts)
+    participant Exec as ExecutionController<br/>(Execution.ts)
+    participant Pool as SASSessionController<br/>(Session.ts)
+    participant FS as Filesystem<br/>(session.path)
+    participant SAS as spawned "sas" process
+
+    Note over Pool,SAS: SESSION CREATION - happens ahead of any request,<br/>pool pre-warms up to 3 sessions (Session.ts:59-69)
+    Pool->>FS: createFile(code.sas, "") - empty dummy SYSIN (Session.ts:117-118)
+    Pool->>FS: readFile(sysInitCompiledPath) - compiled system-init<br/>macros, produced by `npm run compileSysInit` (Session.ts:105)
+    Note over Pool: throws if this file doesn't exist yet -<br/>session creation never reaches execFile below
+    Pool->>FS: createFile(autoexec.sas, autoExecContent) (Session.ts:108-114)
+    Pool->>SAS: execFile(sasLoc, -SYSIN code.sas -AUTOEXEC autoexec.sas -LOG log.log ...) (Session.ts:127-147)
+    activate SAS
+    Note right of SAS: process stays active (this box)<br/>from spawn until it finally exits below
+    SAS->>FS: autoexec step 1: delete code.sas, the dummy SYSIN (Session.ts:257-261)
+    Pool->>FS: waitForSession() polls fileExists(code.sas) (Session.ts:175-192)
+    FS-->>Pool: code.sas no longer exists
+    Pool->>Pool: session.state = pending (Session.ts:190)
+    SAS->>SAS: autoexec step 2: busy-wait for code.sas to reappear,<br/>up to 15 minutes (Session.ts:263-274)
+    Note over SAS: process is now idle, parked mid-autoexec,<br/>waiting on the filesystem.<br/>Counts as "pending" in the pool.
+
+    Note over Client,SAS: REQUEST ARRIVES - reuses the parked process above
+    Client->>Ctrl: POST .../execute { code, runTime: sas }
+    Ctrl->>Exec: executeProgram({ program, runTime: SAS, ... })
+    Exec->>Pool: getSession() returns the pending session (Session.ts:59-69)
+    Exec->>Exec: session.state = running (Execution.ts:96)
+    Exec->>FS: createFile(webout.txt, "") and createFile(reqHeaders.txt, ...) (Execution.ts:103-107)
+    Exec->>Pool: processProgram(program, session, ...) (Execution.ts:110-121)
+    Pool->>FS: createSASProgram() wraps user code with macro vars,<br/>_webout filename, autoexec injection (createSASProgram.ts)
+    Pool->>FS: write code.sas.bkp, then rename to code.sas (processProgram.ts:48-49)
+    Note over FS: write-then-rename, not a direct write,<br/>so SAS never reads a partial file
+
+    FS-->>SAS: code.sas exists again
+    SAS->>SAS: autoexec step 2 loop exits, sleeps 0.01s, autoexec ends (Session.ts:267-271)
+    Note over SAS: -SYSIN has pointed at code.sas from the start,<br/>so SAS now executes ITS CONTENT as the main job
+    SAS->>FS: writes log.log, output.lst, webout.txt while running
+
+    Pool->>Pool: processProgram poll loop:<br/>while session.state !== completed (processProgram.ts:52-58)
+
+    alt program reaches EOF normally
+        SAS->>SAS: exits with code 0
+        SAS-->>Pool: execFilePromise resolves - .then() (Session.ts:148-152)
+        Pool->>Pool: session.state = completed
+        Pool-->>Exec: processProgram() resolves
+    else program aborts (fatal error, license failure, hard STOP)
+        SAS->>SAS: exits with non-zero code
+        SAS-->>Pool: execFilePromise rejects - .catch() (Session.ts:153-163)
+        Pool->>Pool: session.state = failed<br/>session.failureReason = err.toString()
+        Pool-->>Exec: processProgram() throws (processProgram.ts:53-55)
+    end
+    deactivate SAS
+
+    Exec->>FS: read log.log, webout.txt, stpsrv_header.txt (Execution.ts:123, 128-131)
+    Exec-->>Ctrl: { httpHeaders, result }<br/>or throws SessionExecutionError{ message, log } (Execution.ts:109-126)
+    Ctrl-->>Client: 200 + result<br/>or 400 { status, message, error, log }
+```
+
+## Why this design
+
+- SAS has meaningful startup cost (loading the engine, running system init
+  macros). Spawning a fresh `sas` process per request would pay that cost
+  every time. Instead, a process is spawned once and parked, ready to run
+  exactly one job the moment code shows up at a path it's already watching.
+- The pool (`SessionController.getSession`, `Session.ts:59-69`) keeps up to
+  3 such parked processes ready, so most requests get an instant handoff
+  instead of waiting through the ~15-minute autoexec spin-wait or a cold
+  start.
+- A session is single-use: once its process consumes the real SYSIN content
+  and exits (success or failure), that process is gone. The session object
+  itself is deleted later by `scheduleSessionDestroy` (`Session.ts:204-236`),
+  not reused for a second job.

api/docs/diagrams/session-lifecycle.md

@@ -0,0 +1,70 @@
+# Session lifecycle (`SessionState`)
+
+Enum defined in `api/src/types/Session.ts`. A `Session` is `{ id, state,
+path, creationTimeStamp, deathTimeStamp, expiresAfterMins?, failureReason? }`.
+Sessions live in an in-memory array on a per-runtime singleton controller
+(`process.sasSessionController` for SAS, `process.sessionController` shared
+by JS/PY/R — see `getSessionController()` in `Session.ts:239-253`).
+
+```mermaid
+stateDiagram-v2
+    [*] --> initialising: SAS runtime<br/>SASSessionController.createSession()<br/>Session.ts:77-95
+    [*] --> pending: JS/PY/R runtime, no process yet<br/>SessionController.createSession()<br/>Session.ts:30-57
+
+    initialising --> pending: dummy SYSIN file deleted<br/>by SAS's autoexec<br/>waitForSession(), Session.ts:175-192
+    initialising --> failed: spawned SAS process exits<br/>before handshake completes<br/>Session.ts:153-163, 184-188
+
+    pending --> running: session picked for a request<br/>executeProgram(), Execution.ts:94-96
+
+    running --> completed: process exits 0<br/>SAS: Session.ts:148-152 (original process)<br/>JS/PY/R: processProgram.ts:116-120 (fresh process)
+    running --> failed: process exits non-zero<br/>SAS: Session.ts:153-163<br/>JS/PY/R: processProgram.ts:121-131<br/>failureReason = err.toString()
+
+    completed --> [*]: deleteSession()<br/>scheduleSessionDestroy(), Session.ts:194-236
+    failed --> [*]: deleteSession()<br/>scheduleSessionDestroy()
+
+    note right of initialising
+        SAS only. The real sas
+        executable is already
+        running, spin-waiting
+        inside its AUTOEXEC for
+        real code to arrive.
+        See sas-execution-handshake.md
+    end note
+
+    note right of pending
+        Session sits in the pool,
+        reusable. Pre-warmed up to
+        3 pending sessions per
+        runtime (getSession(),
+        Session.ts:59-69)
+    end note
+
+    note right of running
+        SAS: no new process spawned
+        here, request just feeds
+        code to the already-running
+        process.
+        JS/PY/R: a brand new
+        interpreter process is
+        spawned right now.
+    end note
+```
+
+## Consumers of `state`
+
+| Reader | Location | Watches for |
+|---|---|---|
+| `waitForSession` | `Session.ts:175-192` | `failed` (breaks early) or the dummy SYSIN file disappearing (implies session survived init) |
+| `processProgram` (SAS branch poll loop) | `processProgram.ts:52-58` | `completed` (success exit) or `failed` (throws, carrying `session.failureReason`) |
+| `scheduleSessionDestroy` | `Session.ts:204-236` | `running` (extends death timer instead of destroying) |
+
+## Asymmetry between runtimes
+
+- **SAS**: one OS process per session, spawned once at `createSession()`
+  time and reused for exactly one job (see `sas-execution-handshake.md`).
+  `running`/`completed`/`failed` are all driven by that *same* process's
+  eventual exit.
+- **JS/PY/R**: a session is cheap (folder + id, no process). The interpreter
+  process is spawned fresh, per request, inside `processProgram.ts:115` and
+  its exit drives `completed`/`failed` directly in the same function - there
+  is no separate poll loop for these runtimes.

api/src/controllers/code.ts

@@ -114,7 +114,8 @@ const executeCode = async (
       code: 400,
       status: 'failure',
       message: 'Job execution failed.',
-      error: typeof err === 'object' ? err.toString() : err
+      error: typeof err === 'object' ? err.toString() : err,
+      log: err?.log
     }
   }
 }

api/src/controllers/internal/Execution.ts

@@ -15,6 +15,24 @@ export interface ExecutionVars {
   [key: string]: string | number | undefined
 }
 
+// Thrown when the session itself fails (e.g. SAS exits abnormally via
+// %abort;). Carries the complete log - by the time this is thrown, the
+// session's process has already exited, so the log file it wrote is final,
+// not a partial/truncated snapshot.
+export class SessionExecutionError extends Error {
+  constructor(
+    message: string,
+    public log?: string
+  ) {
+    super(message)
+
+    // required for `instanceof` to work when compiling to ES5, since the
+    // default __extends helper does not preserve the prototype chain for
+    // classes extending built-ins like Error
+    Object.setPrototypeOf(this, SessionExecutionError.prototype)
+  }
+}
+
 export interface ExecuteReturnRaw {
   httpHeaders: HTTPHeaders
   result: string | Buffer
@@ -88,18 +106,24 @@ export class ExecutionController {
       preProgramVariables?.httpHeaders.join('\n') ?? ''
     )
 
-    await processProgram(
-      program,
-      preProgramVariables,
-      vars,
-      session,
-      weboutPath,
-      headersPath,
-      tokenFile,
-      runTime,
-      logPath,
-      otherArgs
-    )
+    try {
+      await processProgram(
+        program,
+        preProgramVariables,
+        vars,
+        session,
+        weboutPath,
+        headersPath,
+        tokenFile,
+        runTime,
+        logPath,
+        otherArgs
+      )
+    } catch (err: any) {
+      const log = (await fileExists(logPath)) ? await readFile(logPath) : ''
+
+      throw new SessionExecutionError(err.message, log)
+    }
 
     const log = (await fileExists(logPath)) ? await readFile(logPath) : ''
     const headersContent = (await fileExists(headersPath))

api/src/controllers/internal/processProgram.ts

@@ -50,6 +50,10 @@ export const processProgram = async (
 
     // we now need to poll the session status
     while (session.state !== SessionState.completed) {
+      if (session.state === SessionState.failed) {
+        throw new Error(session.failureReason || 'SAS session failed')
+      }
+
       await delay(50)
     }
   } else {