feat(fuzz): coverage-guided fuzz testing pipeline with cargo-fuzz

.github/corpus/rate_limit/rapid_create

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+dddddddddddddddddddddddddddddddd

.github/corpus/rate_limit/slow_create

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+

.github/workflows/fuzz-test.yml

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-name: Subscription Contract Fuzzing Tests
+name: Cargo-Fuzz Pipeline
 
 on:
   push:
     branches: [main, develop]
     paths:
       - 'contracts/subscription/**'
+      - 'contracts/fuzz/**'
       - '.github/workflows/fuzz-test.yml'
+      - '.github/corpus/**'
   pull_request:
     branches: [main, develop]
     paths:
       - 'contracts/subscription/**'
+      - 'contracts/fuzz/**'
+      - '.github/workflows/fuzz-test.yml'
+  schedule:
+    - cron: '0 6 * * 1'  # weekly: Monday 06:00 UTC
 
 jobs:
-  fuzz:
+  cargo-fuzz:
     runs-on: ubuntu-latest
-    name: Run Fuzzing Tests
+    strategy:
+      fail-fast: false
+      matrix:
+        target:
+          - subscription
+          - pricing
+          - rate_limit
+          - state_machine
+
+    name: fuzz / ${{ matrix.target }}
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
-      - name: Install Rust
-        uses: actions-rs/toolchain@v1
+      - name: Install nightly toolchain (cargo-fuzz)
+        uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
-          toolchain: stable
+          toolchain: nightly
           override: true
-          profile: minimal
+          components: llvm-tools
+
+      - name: Install cargo-fuzz
+        run: cargo install cargo-fuzz --locked
 
-      - name: Cache cargo registry
-        uses: actions/cache@v3
+      - name: Restore seed corpus from cache
+        uses: actions/cache@v4
         with:
-          path: ~/.cargo/registry
-          key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+          path: contracts/fuzz/corpus/${{ matrix.target }}
+          key: corpus-${{ matrix.target }}-${{ hashFiles('.github/corpus/${{ matrix.target }}/**') }}
           restore-keys: |
-            ${{ runner.os }}-cargo-registry-
+            corpus-${{ matrix.target }}-
 
-      - name: Cache cargo index
-        uses: actions/cache@v3
+      - name: Copy seed corpus
+        run: |
+          mkdir -p contracts/fuzz/corpus/${{ matrix.target }}
+          if [ -d ".github/corpus/${{ matrix.target }}" ]; then
+            cp .github/corpus/${{ matrix.target }}/* contracts/fuzz/corpus/${{ matrix.target }}/ 2>/dev/null || true
+          fi
+
+      - name: Run cargo-fuzz (${{ matrix.target }})
+        id: fuzz
+        continue-on-error: true
+        working-directory: contracts/fuzz
+        run: |
+          cargo fuzz run ${{ matrix.target }} \
+            --sanitizer=address \
+            -j 4 \
+            -- \
+            -max_total_time=1800 \
+            -print_final_stats=1 \
+            -artifact_prefix=artifacts/${{ matrix.target }}/
+
+      - name: Upload crash artifacts
+        if: steps.fuzz.outcome == 'failure'
+        uses: actions/upload-artifact@v4
         with:
-          path: ~/.cargo/git
-          key: ${{ runner.os }}-cargo-git-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-git-
+          name: crashes-${{ matrix.target }}-${{ github.run_id }}
+          path: contracts/fuzz/artifacts/${{ matrix.target }}/
+          retention-days: 14
 
-      - name: Cache cargo build
-        uses: actions/cache@v3
+      - name: Upload coverage corpus
+        uses: actions/upload-artifact@v4
         with:
-          path: target
-          key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-build-target-
+          name: corpus-${{ matrix.target }}-${{ github.run_id }}
+          path: contracts/fuzz/corpus/${{ matrix.target }}/
+          retention-days: 7
 
-      - name: Run contract fuzz smoke suite
-        run: |
-          cd contracts
-          cargo test --lib
-          for target in fuzz pricing_fuzz rate_limit_fuzz; do
-            if cargo test --test "$target" --no-run >/dev/null 2>&1; then
-              cargo test --test "$target"
-            else
-              echo "::warning::Cargo test target '$target' is not registered; running workspace tests instead."
-            fi
-          done
-          cargo test --verbose
+      - name: Save updated corpus to cache
+        uses: actions/cache@v4
+        with:
+          path: contracts/fuzz/corpus/${{ matrix.target }}
+          key: corpus-${{ matrix.target }}-${{ hashFiles('contracts/fuzz/corpus/${{ matrix.target }}/**') }}
 
-      - name: Print test results
-        if: always()
+      - name: Notify on crash
+        if: steps.fuzz.outcome == 'failure'
         run: |
-          echo "Fuzzing tests completed!"
+          echo "::error::cargo-fuzz target '${{ matrix.target }}' found a crash!"
+          echo "Download artifacts from: crashes-${{ matrix.target }}-${{ github.run_id }}"
+          echo "To reproduce locally: cd contracts/fuzz && cargo fuzz run ${{ matrix.target }} <crash-file>"

contracts/fuzz/.gitignore

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+# cargo-fuzz artifacts
+corpus/
+artifacts/
+coverage/

contracts/fuzz/Cargo.toml

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+[package]
+name = "subtrackr-fuzz"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.4"
+soroban-sdk = { version = "21.0.0", features = ["testutils"] }
+subtrackr-subscription = { path = "../subscription" }
+subtrackr-types = { path = "../types" }
+
+[workspace]
+members = ["."]

contracts/fuzz/FUZZING.md

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+# Fuzz Testing
+
+This directory contains **coverage-guided fuzz targets** powered by
+[`cargo-fuzz`](https://rust-fuzz.github.io/book/cargo-fuzz.html) (libFuzzer).
+
+## Quick Start
+
+```bash
+# Prerequisite: nightly toolchain + cargo-fuzz
+rustup install nightly
+cargo install cargo-fuzz --locked
+
+# Run all targets (CI does this for 30 min each)
+cd contracts/fuzz
+
+# Single target, quick smoke test (10 seconds)
+cargo fuzz run subscription -- -max_total_time=10
+
+# Full run (30 minutes)
+cargo fuzz run pricing -- -max_total_time=1800
+```
+
+## Target Reference
+
+| Target            | What it fuzzes                                    |
+|-------------------|---------------------------------------------------|
+| `subscription`    | Full lifecycle: create_plan → subscribe → charge → pause → resume → cancel |
+| `pricing`         | Price boundary values, refund math, charge timing  |
+| `rate_limit`      | Per-function rate-limit enforcement windows        |
+| `state_machine`   | Illegal state transitions (double-cancel, charge-while-paused, etc.) |
+
+## Byte Layout
+
+Every target reads a flat `&[u8]` from libFuzzer and parses it as a
+command stream. The first byte selects the action; remaining bytes are
+action-specific parameters (prices, IDs, intervals). See the top comment
+in each `fuzz_targets/<name>.rs` for the exact layout.
+
+## Invariants Checked
+
+After each fuzz action the 10 invariants from `contracts/tests/invariants/`
+are verified:
+
+1. `plan_count_monotonic` — PlanCount never decreases
+2. `subscription_count_monotonic` — SubscriptionCount never decreases
+3. `total_paid_conservation` — total_paid across subs sums correctly
+4. `plan_subscriber_count_accuracy` — subscriber_count matches actual subs
+5. `paused_at_non_zero_when_paused` — paused_at > 0 iff status is Paused
+6. `cancelled_sub_not_chargeable` — cancelled subs can't be charged
+7. `refund_amount_bounded` — refund_requested_amount ≤ total_paid
+8. `next_charge_at_monotonic` — next_charge_at advances forward
+9. `total_collected_non_negative` — total_collected ≥ 0
+10. `user_subs_index_consistency` — every sub in UserSubscriptions exists
+
+## Crash Triage
+
+When a crash is found:
+
+1. **Minimize** the crashing input:
+   ```bash
+   cargo fuzz tmin subscription <path/to/crash>
+   ```
+
+2. **Reproduce** with extra detail:
+   ```bash
+   RUST_BACKTRACE=1 cargo fuzz run subscription <minimized-crash>
+   ```
+
+3. **Classify** — is the crash:
+   - A legitimate contract bug → fix + add regression test
+   - A fuzz-target bug → fix the parser/state setup
+   - Flaky (non-deterministic) → re-run with `-runs=100000` to confirm
+
+4. **Add regression test**: Copy the minimized input to
+   `.github/corpus/<target>/` so the corpus cache prevents regression.
+
+## CI Pipeline
+
+The workflow in `.github/workflows/fuzz-test.yml`:
+
+- Runs all 4 targets **in parallel** via a build matrix
+- Each target runs for **30 minutes** with AddressSanitizer
+- Crash artifacts are uploaded and kept for 14 days
+- Seed corpus is cached between runs for coverage continuity
+- Runs on every push/PR touching contracts and weekly on Monday
+
+### Cache Strategy
+
+The corpus directory is cached per-target, keyed by a hash of the
+checked-in seed files. When a CI run discovers new interesting inputs
+they are saved to the cache for the next run.
+
+## Writing a New Target
+
+1. Add a file `fuzz_targets/<name>.rs`
+2. The file must contain `fuzz_target!(|data: &[u8]| { ... })`
+3. Add a target entry to `.github/workflows/fuzz-test.yml` matrix
+4. Create minimal seed files in `.github/corpus/<name>/`
+
+```rust
+// fuzz_targets/my_target.rs
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| {
+    if data.is_empty() { return; }
+    // parse bytes and exercise contract functions
+});
+```

contracts/fuzz/fuzz_targets/pricing.rs

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+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use soroban_sdk::testutils::Address as _;
+use soroban_sdk::{Address, String};
+use subtrackr_subscription::{Interval, SubTrackrSubscriptionClient};
+
+mod utils;
+
+/// Fuzz pricing edge cases.
+///
+/// Explores boundary price values, refund amounts, and charge timing to
+/// catch math errors (underflow, overflow, rounding) in the pricing engine.
+///
+/// Byte layout:
+///
+/// | Offset | Size  | Field                               |
+/// |--------|-------|-------------------------------------|
+/// | 0      | 1     | action (0=create_plan, 1=charge, 2=refund) |
+/// | 1+     | var   | action-specific parameters           |
+///
+/// Prices are always clamped to a safe range; the fuzzer explores the
+/// boundaries of that range.
+fuzz_target!(|data: &[u8]| {
+    if data.len() < 1 {
+        return;
+    }
+    let mut off: usize = 0;
+    let (env, client, proxy, _storage) = utils::setup_env();
+    let admin = Address::generate(&env);
+    let token = Address::generate(&env);
+
+    loop {
+        if off >= data.len() {
+            break;
+        }
+        let action = data[off] % 3;
+        off += 1;
+
+        match action {
+            0 => {
+                let price = utils::read_price(data, &mut off);
+                let interval = utils::read_interval(data, &mut off);
+                let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+                    client.create_plan(
+                        &proxy,
+                        &proxy,
+                        &admin,
+                        &String::from_slice(&env, b"p-fuzz"),
+                        &price,
+                        &token,
+                        &interval,
+                    );
+                }));
+            }
+            1 => {
+                // charge: advance time so billing applies
+                let sub_id = utils::read_bounded_u64(data, &mut off, 100);
+                utils::set_time(&env, env.ledger().timestamp() + 86_400);
+                let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+                    client.charge_subscription(&proxy, &proxy, &sub_id);
+                }));
+            }
+            2 => {
+                // request/approve refund
+                let sub_id = utils::read_bounded_u64(data, &mut off, 100);
+                let refund_amount = utils::read_price(data, &mut off);
+                // request_refund may panic on invalid preconditions; catch it
+                let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+                    client.request_refund(&proxy, &proxy, &sub_id, &refund_amount);
+                }));
+                let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+                    client.approve_refund(&proxy, &proxy, &sub_id);
+                }));
+            }
+            _ => {}
+        }
+    }
+});