[df] Preserve schema of empty output TTree in Snapshot

[vepadulano]

Previously, the behaviour of Snapshot in case no entries pass the selections in the event loop was two-fold:

• In single thread, Snapshot created an output file with an empty TTree inside, without the user-requested branches.

• In multi-threaded mode, Snapshot created an output file, without any TTree inside.

This commit aligns the behaviour of the two execution modes. Furthermore, it ensures the user-requested dataset schema is preserved even when the output TTree is empty. i.e., all the user-requested branches will be present in the output TTree.

FYI @TomasDado

[github-actions]

Test Results

    18 files      18 suites   4d 1h 11m 35s ⏱️
 2 739 tests  2 739 ✅ 0 💤 0 ❌
47 629 runs  47 629 ✅ 0 💤 0 ❌

Results for commit f5d9d72.

♻️ This comment has been updated with latest results.

[TomasDado]

Thanks! I talked to Vincenzo privately, but I will also post the test results there. There seems to be a crash when JITting the types of the branches in Snapshot.

ROOT Version: 6.35.01
Built for linuxx8664gcc on Mar 31 2025, 07:52:29
From remotes/vepadulano/rdf-snapshot-empty-tree-preserve-schema@a7a87eced7

/home/tomas/ROOT_DEBUG/root_install/include/ROOT/RDF/ActionHelpers.hxx:1636:8: fatal error: instantiating fold expression with 385 arguments exceeded expression nesting limit of 256
       ...);
       ^~~~
/home/tomas/ROOT_DEBUG/root_install/include/ROOT/RDF/ActionHelpers.hxx:1673:10: note: in instantiation of function template specialization 'ROOT::Internal::RDF::SnapshotTTreeHelper<float, float, float, float, float, float, float, float, float, float, int, float, float, float, float, int, float, float, float, float, int, float, float, float, float, int, float, float, float, float, int, float, float, float, float, int, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, int, float, float, float, float, int, float, float, float, float, int, float, float, float, float, int, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, unsigned long long, unsigned int, unsigned int, double, double, double, double, double, double, double, double, double, double, double, double, double, double, double, double, double, double, double, double, double, double, double, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float, float>::SetEmptyBranches<0UL, 1UL, 2UL, 3UL, 4UL, 5UL, 6UL, 7UL, 8UL, 9UL, 10UL, 11UL, 12UL, 13UL, 14UL, 15UL, 16UL, 17UL, 18UL, 19UL, 20UL, 21UL, 22UL, 23UL, 24UL, 25UL, 26UL, 27UL, 28UL, 29UL, 30UL, 31UL, 32UL, 33UL, 34UL, 35UL, 36UL, 37UL, 38UL, 39UL, 40UL, 41UL, 42UL, 43UL, 44UL, 45UL, 46UL, 47UL, 48UL, 49UL, 50UL, 51UL, 52UL, 53UL, 54UL, 55UL, 56UL, 57UL, 58UL, 59UL, 60UL, 61UL, 62UL, 63UL, 64UL, 65UL, 66UL, 67UL, 68UL, 69UL, 70UL, 71UL, 72UL, 73UL, 74UL, 75UL, 76UL, 77UL, 78UL, 79UL, 80UL, 81UL, 82UL, 83UL, 84UL, 85UL, 86UL, 87UL, 88UL, 89UL, 90UL, 91UL, 92UL, 93UL, 94UL, 95UL, 96UL, 97UL, 98UL, 99UL, 100UL, 101UL, 102UL, 103UL, 104UL, 105UL, 106UL, 107UL, 108UL, 109UL, 110UL, 111UL, 112UL, 113UL, 114UL, 115UL, 116UL, 117UL, 118UL, 119UL, 120UL, 121UL, 122UL, 123UL, 124UL, 125UL, 126UL, 127UL, 128UL, 129UL, 130UL, 131UL, 132UL, 133UL, 134UL, 135UL, 136UL, 137UL, 138UL, 139UL, 140UL, 141UL, 142UL, 143UL, 144UL, 145UL, 146UL, 147UL, 148UL, 149UL, 150UL, 151UL, 152UL, 153UL, 154UL, 155UL, 156UL, 157UL, 158UL, 159UL, 160UL, 161UL, 162UL, 163UL, 164UL, 165UL, 166UL, 167UL, 168UL, 169UL, 170UL, 171UL, 172UL, 173UL, 174UL, 175UL, 176UL, 177UL, 178UL, 179UL, 180UL, 181UL, 182UL, 183UL, 184UL, 185UL, 186UL, 187UL, 188UL, 189UL, 190UL, 191UL, 192UL, 193UL, 194UL, 195UL, 196UL, 197UL, 198UL, 199UL, 200UL, 201UL, 202UL, 203UL, 204UL, 205UL, 206UL, 207UL, 208UL, 209UL, 210UL, 211UL, 212UL, 213UL, 214UL, 215UL, 216UL, 217UL, 218UL, 219UL, 220UL, 221UL, 222UL, 223UL, 224UL, 225UL, 226UL, 227UL, 228UL, 229UL, 230UL, 231UL, 232UL, 233UL, 234UL, 235UL, 236UL, 237UL, 238UL, 239UL, 240UL, 241UL, 242UL, 243UL, 244UL, 245UL, 246UL, 247UL, 248UL, 249UL, 250UL, 251UL, 252UL, 253UL, 254UL, 255UL, 256UL, 257UL, 258UL, 259UL, 260UL, 261UL, 262UL, 263UL, 264UL, 265UL, 266UL, 267UL, 268UL, 269UL, 270UL, 271UL, 272UL, 273UL, 274UL, 275UL, 276UL, 277UL, 278UL, 279UL, 280UL, 281UL, 282UL, 283UL, 284UL, 285UL, 286UL, 287UL, 288UL, 289UL, 290UL, 291UL, 292UL, 293UL, 294UL, 295UL, 296UL, 297UL, 298UL, 299UL, 300UL, 301UL, 302UL, 303UL, 304UL, 305UL, 306UL, 307UL, 308UL, 309UL, 310UL, 311UL, 312UL, 313UL, 314UL, 315UL, 316UL, 317UL, 318UL, 319UL, 320UL, 321UL, 322UL, 323UL, 324UL, 325UL, 326UL, 327UL, 328UL, 329UL, 330UL, 331UL, 332UL, 333UL, 334UL, 335UL, 336UL, 337UL, 338UL, 339UL, 340UL, 341UL, 342UL, 343UL, 344UL, 345UL, 346UL, 347UL, 348UL, 349UL, 350UL, 351UL, 352UL, 353UL, 354UL, 355UL, 356UL, 357UL, 358UL, 359UL, 360UL, 361UL, 362UL, 363UL, 364UL, 365UL, 366UL, 367UL, 368UL, 369UL, 370UL, 371UL, 372UL, 373UL, 374UL, 375UL, 376UL, 377UL, 378UL, 379UL, 380UL, 381UL, 382UL, 383UL, 384UL>' requested here
         SetEmptyBranches(fInputTree, *fOutputTree, ind_t{});
...

[TomasDado]

Thanks for the updates! With the latest changes my code finishes properly and I also see the branches in the output of a Snapshot that has no events passing in a multithreaded mode.