Execution Flow

graph TD
    A[Start: maybe_fetch] --> B{Offline mode?}
    B -->|Yes| C{Dep already exists?}
    B -->|No| D[download_source]

    C -->|Yes| E[Use cached version]
    C -->|No| F[Error: offline, no cache]

    D --> D1[Resource type detection]
    D1 --> D2{Source type?}

    D2 -->|pkg| G[Hex Package Download]
    D2 -->|git| H[Git Clone/Fetch]
    D2 -->|hg| I[Mercurial Clone]

    G --> G1[Check local cache]
    G1 --> G2{Cached with valid ETag?}
    G2 -->|Yes| G3{ETag matches server?}
    G2 -->|No| G4[Download from Hex]

    G3 -->|Yes| G5[Use cached tarball]
    G3 -->|No| G4

    G4 --> G6[r3_hex_repo:get_tarball]
    G6 --> G7[Verify checksum]
    G7 --> G8{Checksum valid?}
    G8 -->|Yes| G9[Store tarball in cache]
    G8 -->|No| G10[Error: bad checksum]

    G9 --> G11[Extract tarball to TmpDir]
    G5 --> G11

    H --> H1{Git version check}
    H1 --> H2{Branch, Tag, or Ref?}
    H2 -->|Branch| H3[git clone -b branch --single-branch]
    H2 -->|Tag| H4[git clone -b tag --single-branch]
    H2 -->|Ref| H5[git clone + checkout ref]

    H3 --> H6[Clone complete]
    H4 --> H6
    H5 --> H6

    I --> I1[hg clone]
    I1 --> I2{Tag or revision?}
    I2 -->|Tag| I3[hg update tag]
    I2 -->|Revision| I4[hg update -r rev]

    I3 --> I5[Clone complete]
    I4 --> I5

    G11 --> J[Move TmpDir to FetchDir]
    H6 --> J
    I5 --> J
    E --> J

    J --> K[Read dep's rebar.config]
    K --> L[Update app info with config]
    L --> M[find_app: Discover application]

    M --> M1{Application found?}
    M1 -->|Yes| N[Mark as available]
    M1 -->|No| O[Error: dep_app_not_found]

    N --> P[Return updated AppInfo]

    style A fill:#e1f5ff
    style P fill:#e1ffe1
    style F fill:#ffe1e1
    style G10 fill:#ffe1e1
    style O fill:#ffe1e1

Detailed Steps

1. Fetch Decision (maybe_fetch/5)

   • Check if dependency already exists locally
   • Check if update is needed
   • Skip if checkout dependency (already local)
   • Decide whether to fetch or use cached version

2. Source Type Detection

   • Determine resource type from source tuple:
     • {pkg, Name, Version, _, _} → Hex package
     • {git, URL, RefSpec} → Git repository
     • {hg, URL, RevSpec} → Mercurial repository

3. Offline Mode Handling

   • If offline mode and dependency not cached: error
   • If offline mode and dependency cached: use cache
   • Otherwise proceed with download

4. Package Download (Hex) (rebar_pkg_resource:download/4)

   • Check local cache directory for existing package
   • Read ETag file to get cached ETag
   • Request package from Hex with If-None-Match header
   • If 304 response: use cached tarball
   • If 200 response: download new tarball
   • Verify checksum against registry
   • Store tarball and ETag in cache
   • Extract tarball to temporary directory

5. Git Clone (rebar_git_resource:download/4)

   • Detect Git version for optimal clone command
   • For branches: git clone -b branch --single-branch
   • For tags: git clone -b tag --single-branch
   • For refs: git clone + git checkout ref
   • Clone to temporary directory
   • Handle authentication if required

6. Mercurial Clone (rebar_hg_resource:download/4)

   • Use hg clone to clone repository
   • Update to specific tag or revision
   • Clone to temporary directory

7. Extraction and Placement

   • Create temporary directory with ec_file:insecure_mkdtemp/0
   • Download/extract to temporary directory
   • Remove old cached version if exists
   • Move temporary directory to final location
   • Final location: _build/PROFILE/lib/DEPNAME/

8. Post-Download Verification

   • Read dependency's rebar.config
   • Update app info with dependency's configuration
   • Discover application structure with rebar_app_discover:find_app/4
   • Verify application is valid
   • Mark application as available